If performers see l’istesso tempo in a piece of music, they know they should perform a passage at the same speed as the previous one, even though the time signature changes.
L’istesso tempo is Italian for "the same speed." Sometimes a piece of music changes time signature such that the same basic rhythm is expressed in more or fewer notes. The time signatures 2/4 and 6/8 both have a two-beat rhythm, but in 6/8 the two beats are divided into three eighth notes each. It sounds faster because the notes are more crowded. If a piece changes from 6/8 to 2/4 but says l’istesso tempo, the two beats should keep coming at the same speed.