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.