Metroidvanias, named after the popular Nintendo franchise Metroid, are 2D platformers that are mostly built on their tough, unforgiving bosses and exploration.
Typically in Metroidvanias one of your main goal is to gain new abities, these can be anything from dashing to wall climbing and when you unlock more and more abilities new areas open up.
For example in the first area of the game there might be a large gap that without the dash ability the player cannot cross but when they ger the dash aility then they have to go back to that first area.