A Micro Filter (also known as a Splitter) is a small device used to split voice (telephone) and data (internet) signals.
A Micro Filter plugs into the telephone wall socket and has a socket for the telephone plug to go into and also a smaller one to fit an RJ-11 plug that goes to the modem/router.

It is important to add a Micro Filter to all the telephone sockets that share the same line as the broadband that also have a telephone attached to them. If you use the telephone for an incoming call or if you are making a call through a socket that doesn't have a micro filter then the Internet connection can be interrupted.

If you have a socket that does not get used and has no telephone plugged into it then should not need a micro filter added to them.

Just to note that when you order broadband you will most likely receive 1 or possibly 2 micro filters from your broadband provider. If however you need more then they are relatively cheap and only cost £6-£10.