By default, vim uses absolute line numbering. This can be turned off with set nonumber or more concisely set nonu. Turn it back on with set nu. Get more details at :h number.

Vim also supports relative line numbers. If you’d rather use relative line numbers, first turn off absolute line numbers (set nonu) and then turn on relative line numbers with set relativenumber. Shave off some characters with set rnu. As you might expect, you can turn off relative numbering with set nornu.

See :h relativenumber for more details.