What does a back-to-back contract mean for a subcontractor?

It is usually done with a clause saying you are bound by the main contract so far as it relates to your work. Some add a promise not to put your main contractor in breach of it. In practice the two contracts rarely match exactly, because your main contractor keeps time for itself. So a 14 day notice period in the main contract can become 7 days in your subcontract. Yet its liability to the client for delay can come down to you in full. Whether general words like these bring in every term of the main contract, or only some, depends on the wording. That is a question for your solicitor. Commercially, ask for the main contract before you sign and read the parts that reach your work. Then set the periods and liabilities it passes down against the ones your subcontract gives you.