What is this thing?

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"The thing is the big rectangular plastic thing with the hoses going into it. The problem I am having is that the round thing with the hose going to it is held by 2 metal brackets, however one of mine has somehow fallen of and now it just bounces around in there."

Rectangular item is the carbon canister that retains fuel vapors that are eventually returned to the engine for consumption. The round thing is the canister vent valve. A normally open valve that allows outside air to enter the canister.
What happens is there is a purge solenoid that opens per instructions from the PCM and this allows fuel vapors to flow from the canister to the intake manifold. If there was no vent to allow in fresh air, the canister would collapse.
If you can't make it to the dealer, see if it can be tie strapped