In a fugue, this refers to a phrase in one voice being closely followed by the same phrase in another voice, while still fitting together contrapuntally.
(The term stretto means “tight” or “close” in Italian.)
For example, in the Fugue-Kriti, a stretto is used between the violin and veena in mm. 75–76 of the Charanam (Movement III; measure numbers appear at the top of the video.)

Example: Stretto