Learn German: Das Leben überrascht einen eigentlich immer wieder. - Life actually surprises you again and again.

man is a subject pronoun. In this sentence, Das Leben (life) is the subject doing the action. The verb überraschen (to surprise) requires a direct object in the Accusative case. The Accusative form of the general pronoun man is einen.

The translation says 'you', but doesn't einen mean 'a' or 'one'?

It does! einen is the Accusative form of man ('one' or 'people in general'). In English, we usually say 'Life surprises you' to talk about people in general. But in German, you cannot use dich (you) for this general meaning; you have to use einen.

Why does einen come right after the verb, before eigentlich?

In German sentence structure, pronoun objects are light, so they tend to slide as close to the conjugated verb as possible. That is why einen comes before the modal particle eigentlich and the time phrase immer wieder.

What is the exact meaning of immer wieder here?

Literally it translates to 'always again', but as a fixed expression it means 'again and again', 'time after time', or 'constantly'.