Before locked, MsMs is referring to a screenshot / video that acts as evidence of their SteamID to prove they were in the game at the time the incident occurs, you can obtain this by pressing tab, clicking their profile and press F12.

Before locked, MsMs is referring to a screenshot / video that acts as evidence of their SteamID to prove they were in the game at the time the incident occurs, you can obtain this by pressing tab, clicking their profile and press F12.