In the Japanese martial arts ukemi is a section of training in skills where one learns to land on the ground without getting hurt. As always there are many layers that build on it, the first being just learning how to fall safely- have you ever been taught how to fall and land on the ground safely?
From here it builds to being able to roll, recover, and get back up in kamae- martial arts posture.
In training ukemi is important for two reasons- the first that it allows one to practice without getting injured, and the second as it allows one to *feel* the application of the technique, to feel the transmission of the training.
An important fundamental skill (kihon) that is often required for black belt level and training in the martial arts.