Over the years, Audacity has changed the storage location of its plug-ins in order to facilitate easier install and uninstall. This is a good thing, but in the internet age- when easily accessed instructions often outlive their usefulness- it can create confusion, conflict, and massive headaches for new and advanced Audacity users alike.
The first step to reducing noise in Audacity is telling the application exactly what you consider “noise,” so that it doesn’t reduce or remove any actual content.
The overgeneralized way of explaining compression is that it raises the volume of quiet things and lowers the volume of loud things. The advantage of applying compression is that everything will tend towards a consistent volume, which is listener-friendly; no one wants to strain to hear certain things and then have their eardrums blown out during subsequent loud sequences.