Hey Francesca,

to give rep there is a little star at the bottom of each post
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You click that star and a message pops up for you to rep someone and to leave a comment.
Rep comments are only seen by the recipient. You don't have to leave a comment.
Most do, but sometimes when I am very busy I may not. But I try to leave a comment each time.

When you get the message that you must 'spread rep' before you can rep the same person again,
it means that you need to rep more people first. I'll post and example.
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That's what it showed when i tried to rep you. Because...
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You were the last person I repped.
So I would need to rep another 9 or 10 people before I can rep you again.
You can only rep 10 people in a day though.

As you gain rep, your rep power goes up and you give more rep to each person.
Most people usually only rep a map post they like, but if someone posts a very helpful comment, that can be repped too.