Depends on the type of athlete, but in general they would tend to eat a diet high in protein and high-ish in complex carbohydrates and low in fat. They also need to make sure they keep their “electrolytes” balanced. These are elements we take in, in our diet that keep say your heart beat running correctly. And when you sweat a lot or are working out a lot you’ll lose a lot of these so they often will take a solution to replace these.
To add to John’s great answer, depending on the athlete they will also have different calorie requirements. An average man needs about 2500 calories and an average woman needs around 2000 calories a day. But bodybuilders for example will need a lot more, closer to 3000 for a man.
I read somewhere recently that Dwayne Johnson (The Rock) eats a massive 5000 calories a day!