Other forms: rubatos
In a rubato passage or piece of music, a performer can exercise some freedom with the tempo. They shouldn't lag behind or rush ahead of the main beat, but they don't have to sing each note right on time as written.
Rubato is short for tempo rubato in Italian, which literally means "stolen time." That's because any particular note might be held longer than written, but in order not to fall behind when doing this, the performer must "steal" time from other notes, singing them more quickly than written or not holding them as long. Singing or playing rubato — a bit faster here, and a bit slower there — allows the performer to add some interest and personal expression to the piece, so it doesn't sound mechanical.