Long story short, every user has some ability (well, almost every user) to affect another user's reputation positively or negatively by 'voting' them up or down in the posts they make.
The amounts that users can affect someone's reputation, per vote, are what you're seeing there. There are some cases (brand new user and a couple of others) where they have 0 ability to affect someone's reputation and there are many cases (veteran or more active users) where they can affect it moreso.
There are also some forums (for instance the feedback forum) where votes do not affect another person's reputation no matter how much ability you have to do so in the other forums.
An explanation of how the numbers work can be found here as well as here. As for how to actually vote the reputation, you should be able to see next to any post (on the upper right) a number with an up and down arrow next to it (the arrows won't appear for your own posts). By clicking one of those arrows you are voting that post up or down. You would also be prompted to affect the user's reputation (with a comment) at the same time. One thing to note is that there are also sub-restrictions on reputation voting that prevent you from excessively giving/removing reputation from the same user over and over.