Full Idea
A 'prefix' is a finite sequence of positive integers. A 'prefixed formula' is an expression of the form σ X, where σ is a prefix and X is a formula. A prefix names a possible world, and σ.n names a world accessible from that one.
Gist of Idea
The prefix σ names a possible world, and σ.n names a world accessible from that one
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.48