Sequencer gives users the ability to create in-game cinematics through its specialized multi-track editor. By creating Level Sequences, adding tracks, and creating keyframes you can manipulate objects, characters, and cameras(Unreal Engine Community, 2022).
The concept of the sequencer in UE is basically equivalent to the combination of camera and character animation, which coordinates the planning of shot and timeline.
The above link is a detailed explanation from EPIC officials about the concept of sequencer.
Unreal Engine 5 : Deep Dive Into UE5 Sequencer | Epic Developer Community
For more details, answers can be found in the 2 UE community posts links attached above.