Email segmentation just means splitting your list into groups so you can send each group something that matters to them.
A new enquiry wants different things from a customer who’s bought from you five times. Someone in Stroud cares about your open day. Someone in Manchester doesn’t. When everyone gets the same email, most of it misses.
You don’t need dozens of groups or fancy software. Three or four sensible groups, and a clear idea of what each one needs to hear, will make a real difference to how many people open, click and buy.