Campanella ('little bells') is method of playing consecutive notes on different strings as far as possible, so that each plucked note rings into the next one. It exploits the re-entrant tuning.
Examples from Ukulele Hunt:
This is standard fingering
and this is campanella fingering.
There are several threads on this forum, e.g. a demo here (second post) and a downloadable guide here.
I find it quite difficult to do but love the effect.
Latecomer to uke but loving it! Baton Rouge U108S soprano, Kiwaya KS-5 soprano, Uluru II concert low G, Barnes & Mullins banjolele UBJ2.