A concertino, literally “little ensemble”, is the group of soloists in a concerto grosso. This is opposed to the ripieno and tutti which is the larger group contrasting with the concertino.

Though the concertino is the smaller of the two groups, its material is generally more virtuosic than that of the ripieno. Further, the concertino does not share thematic material with the ripieno, but presents unique ideas. This contrast of small group to large group and one thematic group against another is very characteristic of Baroque ideology—similar to terraced dynamics where the idea is significant contrast.

https://en.wikipedia.org/wiki/Concerto_grosso#Concertino