This command only applies to Azure SQL Database, at a high level it empties the database authentication cache for logins and firewall rules for the current USER database.
In Azure SQL Database the authentication cache makes a copy of logins and server firewall rules which are in the master database and puts them into memory within the user database. The Database Engine attempts re-authorisation using the originally submitted password and no user input is required.
If this still doesn’t make sense, then an example will really help.
Click through for the helpful example.