s is the substitution command, the
% before it means that all lines of the file will be considered.
, is the separator. It is common to choose
/ as the separator, but since there are slashes in the pattern and replacement, it is useful to choose another separator, so as to avoid escaping.
\v enables very magic mode, so that special symbols
+() do not need to be escaped.
(\d+) is the a capture group, capturing digits.
g indicates that all matches in a line should be replaced, not only the first match.
The substitution is done by reordering the groups as
Sample input before
12/11/1212 VIM 10/1/1
and after the command
1212/12/11 VIM 1/10/1
You can find more on the substitute command at