Welcome to the community!
If you are doing the trade-in online once the trade is accepted, the value will appear as a bill credit on your Fido account within three billing cycles.
If you are doing it in-store the credit will be applied towards the purchase price of a new phone. If the phone costs less than the value of the trade, you can use the rest of the credit towards accessories. The credit will be applied immediately in-store.
You can find out more about the trade-in program here.
As for your tablet if you are done paying off the current one you can definitely get another or you can pay off the balance of the current tablet and get another if you like.