Feats Rule: System Reference Document
A feat represents a talent or an area of expertise that gives a character special capabilities. It embodies training, experience, and abilities beyond what a class provides.

At certain levels, your class gives you the Ability Score Improvement feature. Using the optional feats rule, you can forgo taking that feature to take a feat of your choice instead. You can take each feat only once, unless the feat’s description says otherwise.

You must meet any prerequisite specified in a feat to take that feat. If you ever lose a feat’s prerequisite, you can’t use that feat until you regain the prerequisite.

Netbook of Feats Commentary on the Feats Rule
All of the core classes receive Ability score improvements at levels 4, 8, 12, 16, and 19 giving an opportunity for 5 feats to be taken by a character. Note this means feats are not something character’s typically start out knowing, they are skills they learn while adventuring or possibly between adventures.

Likely if you choose to use the optional rule in your campaign, your players will take advantage of it. A number of feats grant you an Ability Score Improvement as well as a few other small benefits thus such feats are almost always mechanically superior to just taking an ability score increase. Those that don’t raise an ability score offer benefits that may or may not be superior, such things can be hard to judge.