Straining means passing food through a sieve in order to remove undesired solid components such as seeds. The sieve is usually very finely woven and thus filters even the smallest residues. You can even use a cloth strainer, like a cheesecloth, to filter the cloudiness from a broth, but you will probably need to repeat the process more than once. If you strain frequently, it might be worth getting a food mill for easy straining at home.
The technique is used to create a great variety of dishes from sauces to purées and pulps. Clear beef broth, seedless jam, and silky tomato sauce are only a few products whose signature textures are obtained via straining.