The search allows you to filter in different ways.

1. Keyword Search

Searching by keyword allows you to find people based on the keywords that they have in their profile. For example, you are are a keen swimmer, you may want to find others who also love to swim. Starting typing your keyword in where indicated by the red arrow. If there is anybody with that hobby or interest, that keyword will appear and you can select it.

Once you have selected your keyword, it will appear below (as indicated by the red arrow in the screenshot below). 

The users in the search results will match that keyword. Looking at that person's profile will show that they have that hobby or interest (as shown below).

You can delete that keyword if you wish, or add new ones. If you have multiple keywords in your search filter, users will match if they have at least one of them.