Watch-only wallet means the wallet only has access to its public key (usually "extended public key" or "xpub"). This allows the wallet software to monitor any activity in that wallet (receiving and sending) without being able to spend the funds.
What this warning is saying is that unless you have the private key to that wallet somewhere else, it isn't actually your wallet. If this public key was given to you by your "trader", I'm sorry to say that you have been scammed. They're only trying to make it seem like you have access to your funds. You should probably check out this question.
If you do have your private key somewhere else – in a different wallet that you used to generate this public key and then imported the public key into Trust Wallet – then there's nothing to worry about. Your funds are still accessible from the previous wallet as long as you have it.