When a consonant that's already pronounced simply slides onto the next word — no new sound, just smoother speech.
People lump enchaînement in with liaison, but they’re different mechanisms.
The difference
In liaison, a silent consonant wakes up. In enchaînement, a consonant that’s already pronounced just resyllabifies onto the next vowel.
il arrive → i‿l‿arrive. The l was always there; it simply leans
forward to start the next syllable.
Why you should care
Enchaînement is a big part of why spoken French sounds like one long ribbon instead of separate words. Train your ear for it and listening gets much easier.