Meaningful Skills are all the "things" the player must do to take a challenge from its beginning state to its completion state. In other words, once the player has recognized a challenge's objective, the work has only begun.
It can't be stressed enough that I'm referring to meaningful skills. "Meaningful" is an incredibly important part of this equation. If a skill is too basic, it will not help make your mechanic feel deeper. At that point, it becomes a simple task the player must complete, like checking items off a shopping list.
A classic example of a too-basic skill can be found in the statement "move from point A to point B." That kind of fundamental movement challenge is essentially the same as saying "hold a button on the controller to win." It's foundational, but it's too simple. It's not deep.
Further, when you really think about it, when you say "move from point A to point B," you're actually talking more about the objective of a challenge and not the skill required to achieve the objective.
This sounds like a small distinction, but is in fact very important. Making this shift in how you think about designing game mechanics allows you to see depth-related problems that you otherwise would not.
When I was a junior designer on Ratchet & Clank 2, I was given the task of coming up with all the "Tractor Beam" puzzles for the two levels of the game that used them. The tractor beam was a game mechanic that allowed Ratchet to freely move large objects marked with a special tractor beam symbol. Essentially this is a theatrical "paint-over" of classic block-pushing challenges from games like The Legend of Zelda.
Coming up with the training setups was easy. The player simply had to move blocks from point A to point B so that he could jump up on them and get out of a pit. My problems began, however, when I tried to come up with more advanced challenges. I quickly came to an impasse. Moving blocks from one place to another, I saw, was too basic a skill. Beyond the training example, there wasn't much more I could do to "ramp it up" and provide varied, escalating challenges.
At this time in my career, I didn't yet understand the important distinction between meaningful skills and too-basic skills. I didn't know how important clearly identifiable objectives were. And so, lacking experience, I decided to just start adding features until the mechanic was deep enough. If you're groaning at this, then I congratulate you. I'm groaning, myself, as I write this.
Besides moving blocks around, I decided it would be great if the player could grab and drag around a wacky robot (if you're reading this and thinking "oh, you improved the theatrics," you get a cookie). The player could then drop the robot on buttons to open doors. This didn't help as much as I wanted it to -- it just still seemed way to shallow.
So I forged ahead and kept adding features (groan).
I added bombs that you could drag around to blow up doors and little energy slingshots you could use to fling the bombs around at targets.
I decided blocks would be able to get in the way of laser beams so you could get past them.
I added special blocks with explosive rockets inside that would blow up certain doors.
Finally, I made it so that some of the blocks slid around inside a groove on the floor. The player had to figure out how to arrange the blocks into a specific order, but the blocks all got into each others way, since they shared a groove.
By the end of all that adding, not only was I permanently on several programmers' hate lists, but the tractor beam game mechanic was quickly getting too bloated. It was way too complicated for players to handle. Playtests were generating feedback that players were routinely confused. We had to devote about half the time spent with the tractor beam purely to training, so that our players could understand the basic gist of what they needed to do.
Looking back, it's clear to me why the mechanic never felt deep enough: I kept adding new objectives, but failed to add many meaningful skills.
I'll go into this a little more below, but I bring it up now because it illustrates how meaningful skills contribute much more to a feeling of depth than objectives do.
Experiences like the one I had with the tractor beam taught me a valuable lesson: most game mechanics that don't feel deep enough feel that way because they have too many objectives and not enough meaningful skills.
While players found the Inspector Bot wacky and funny, adding him did not succeed at the goal of making the Tractor Beam game mechanic deeper.