Under your bonnet, there is a pipe going from your crankcase into your air intake. The purpose of this is to allow unburnt fuel and oil vapours that get past the pistons to be recycled and burned in the engine. These vapours get sucked into the engine via the carburettor (where air would normally go in).
By replacing this method with a crankcase breather, you are allowing the gasses and vapours to run through a filter and into the atmosphere instead of back into your engine.
This will allow your car to breathe more of the oxygen it needs instead of unburnt fuel and contaminated oil vapours and as we all know, the more oxygen your engine gets, the better it will perform.
Crankcase breathers are available for around £5-£6 and most are universal and will fit most vehicles. 15-30 mins fitting time