A tier is a level or a layer. If you sit in the top tier of seats at a concert, you can see the whole stage and most of the audience — but you probably have to climb lots of stairs.
A tier can describe a layer of ruffles on a dress, a section of seats in a stadium, or one layer of a fancy cake. You could also talk about a tier in terms of value or position. For example, the most popular, well-known actors might be considered part of the top tier, getting all the best parts. Tier originated from the Old French word tire: rank, sequence, or order.