To comment on the same line as the code you can use &:: or &rem. You can also use && or || to replace &.
Example :
@echo off
echo This is a test &::This is a comment
echo This is another test &rem This is another comment
pause
A curiosity: SET command allows limited inline comments without &rem:
set "varname=varvalue" limited inline comment here
Limitations:
set "varname=varvalue" or set "varname=",cmd poisonous characters | < > & must be properly escaped as ^| ^< ^> ^&,( ) must be properly escaped as ^( ^) within a bracketed code block.