This does not, however, pertain to the kind of hat a player is donning. Instead, it is a phrase used to refer to their senior national team.
Let's explore the origins of the saying and how it is now used:
In soccer, a player's number of caps is determined by the number of appearances they have made with their senior national team; their club is unrelated to this figure.
According to legend, the phrase first appeared in the United Kingdom, where athletes would receive actual caps to signify their participation in international competitions.
Even though there are no longer any physical caps, it is still one of the greatest achievements in the game.
Only by competing in an international game can a player earn a cap. A player must play in a specific game for their minutes to count, so simply being listed to the squad is not sufficient.