What is this? Where does it go. I think I accidentally disconnected it while reaching inside my engine. To grab a but that had fallen

 

Is that the front mount tube. Hard to tell

 

Looks like a vacuum hose to an engine mount.

 

The OP didn't specify year; but, that is the vacuum assisted rear engine mount for models with automatic transmission.

One end of the hose goes to the rear engine mount, the other end of the hose goes to the engine mount control solenoid valve. This other thread shows a photo of the solenoid valve end. https://www.hondaaccordforum.com/for...line-go-60702/

It is used to control excessive vibration at low rpms (e.g. idle). At low rpms, the PCM sends a ground signal (-) to the engine mount control solenoid valve, which opens the valve.