This video tells you all you need to know about what a cTrader ID (cTID) is.

In brief, a cTID is a profile that allows you to access the entire cTrader ecosystem using just one set of credentials. Trading accounts registered with different brokers can all be linked to your cTID, making it possible to change between these accounts in non-branded cTrader applications. In a cTrader application of a specific broker you will only see accounts registered with this broker.