cTID is “cTrader ID”.
It is an unique ID for cTrader users, and each ID links to cTrader trading accounts. One ID can link to multiple accounts.
You can have a “cTrader ID” for yourself once you create one in the official website of cTrader. (or on the website which opens when you open cTrader trading platform)
By registering a cTrader ID, you can access to all your accounts by just logging in one time on the platform.