Rigidbody2D is Unity’s 2D physics body component. It lets a GameObject use velocity, gravity, forces and 2D physics callbacks. In beginner projects, it is commonly attached to the moving player so Collider2D and trigger events fire reliably. (course lab series, see source-csharp-unity-labs)
Physics movement should usually be applied in FixedUpdate, while player input is read in Update.
Related: unity-rigidbody2d, collider2d, trigger, transform-vs-rigidbody2d-movement