What is Multiplet, n+1 rule?

A signal is often split into several close lines: a multiplet (singlet, doublet, triplet, quartet, … for 1, 2, 3, 4 lines). By the $n+1$ rule, a group of equivalent protons whose neighbouring carbon atoms carry $n$ protons in all, not equivalent to it, gives a signal of $n+1$ lines, with intensities in the ratios of Pascal’s triangle.