Usually found in 2D shooters and platformers, auto-scrolling levels progress of their own volition, regardless of where the player is or what they are doing. It's up to the player to keep up which makes these levels challenging. To start, they require quick reflexes since you're not advancing at your own rate, obstacles can pop out at any moment, hitting the edges of the screen will sometimes kill you, jumps can be hard to make and you can get trapped by geometry that has already been cut from the frame. Often, the scrolling speed may suddenly quicken, or slow down during a level, especially when you are mid-jump above razor spikes.
Auto-scrolling levels aren't simply limited to horizontal scrolling and will scroll vertically (sometimes within the same level, but not usually). In early 3D games like Crash Bandicoot, auto-scrolling levels usually took the form of a chase, like a giant stone ball chasing you or lava burning the path away behind you. In fact, auto-scrolling level tactics haven't really changed at all since then.