A filter is made of a material with very many fine holes in it. You probably know of many filters in your home, you just may not think of them.

A tea bag is a filter. It stops the powdered tea from getting into the water. If you still use loose tea, then a tea strainer is a filter you would use.

A vacuum cleaner has a filter built in to stop the dust from being sucked in one end and blown out of the other.

A flour sieve is a filter.

Motor cars have oil filters to stop bits and pieces of solid getting into the engine where they could cause harm.