If you come across a post that you find helpful or answered your question, or you simply like someones post, you can give him or her a thumbs up, called a Like. You will find this at the end of every post:
If someone posted the solution to your problem in the topic you've started, you should click on "Mark as solution":
This rewards the person that solved your problem, and at the same time other users will find this solution in case they have the same issue.
Edit: You can now mark solutions from mobile devices as well.
All the best,