Any skill. At first glance, this just makes Crash Course better than a card like Consultant Call. However, note a few subtle differences.
First, Crash Course requires a

to stick the skill on.
Second, you can't give a

a skill it already has (though you can improve it to expert). This is relevant because there may be instances where you'd like two tokens of the same skill, so going from trained to expert won't really help.
Third, Crash Course only provides the skill for the phase, rather than the round. So if you want a token, you have to play it during the Resource Phase. However, there are other uses if you play it in other phases. For instance, it can also allow you to play heroics for skills your crew natively lacks if you use it during the Action Phase