What is the difference between pm2 restart and pm2 reload

The difference is documented here:

As opposed to restart, which kills and restarts the process, reload achieves a 0-second-downtime reload.

The latter means (found here):

With reload, pm2 restarts all processes one by one, always keeping at least one process running.

It also states that:

If the reload system hasn’t managed to reload your application, a timeout will fallback to a classic restart.