1. To let it go; to start it up. Often used as an imperative. "Her" is used in the same way that some ships and machines are referred to as female. OK, the rocket is ready to launch. Let her roll! I replaced the spark plug, so go ahead and let her roll so we can see if there's any difference. We've spent so long working on this ad campaign that I'm excited to finally let her roll.

2. To do something without inhibition or restraint, typically with great enthusiasm or force. Wow, did you see that kick? He really let her roll. When I'm writing a first draft, I like to just let her roll and not worry about typos or grammar.