what does a" start box "mean
 

I just see a chart in the CYUL chart sheet named “start boxes” sorry for my ignorance ,but what is the definition of start boxes?

Because of limited space and ground equipment, the airport authority have established safe spots for engine start on the apron area, that's all. Start boxes are where you start your engines after push back.

Keep in mind that not all aircraft at YUL start engines in the Start Boxes. Only aircraft parked at some gates.

Other airports have similar but different names. In RPLL, they are called Start-Up points and are shown as circles on the Jepp chart. ATC tells you which one to use and you tell the ground crew. Chart 10-9B for VIDP shows numbered "Tug Disconnect Points" which is really just a nosewheel size painted area where you are told to start.