Finger clusters (or clustering) is a technique whereby once you have made a ‘cluster’, don’t break that cluster if you will then use those same fingers (usually) in the same order, immediately afterwards. It’s a pretty basic technique, sometimes taught in the early stages, depending on the teacher.
The simplest example would be on fiddle, playing a simple ascending one-octave scale starting on open D. The fingering is 0-1-2-3 (on the D string), then 0-1-2-3 (on the A string). On your first 0-1-2-3, you keep that ‘cluster’ that you have made, then lift the fingers *as a unit*, move your hand slightly and repeat the 0-1-2-3 on the A string. Two advantages – maximum economy of movement, and if done properly will preserve your intonation.
If you read the music for this simple scale, you will read D-E-F#-G as note cluster 1 (finger cluster 1), then A-B-C#-D as note cluster 2 (also finger cluster 1).