I need to replace the rear engine mount on my accord I was wondering why it has a vacuum hose on it and what's it for

 

For Automatics the rear mount uses a vacuum controlled diaphragm hydraulic mount. When the car is idling in gear, or a heavy load(A/C) is placed on the engine, the mount disconnects and floats the engine to reduce vibrations into the cabin.

 

Since its bad would there be any kind of jerking or surging going down the road

 

If the rubber portion of the mount broke, hydraulic fluid chamber was ruptured, internal diaphragm no longer worked, or the vacuum controls failed the mount would not stiffen up under power/cruise and would allow the engine to flop about. Check the front engine limiter mount(dogbone) this place a significant part when loading up the mount, if it is damaged it will allow excessive movement as well.