I went under the truck I just recently purchased and noticed this big gaping hole... I am by no means mechanically inclined and wanted to know what the hell is going on.. was something suppose to be there like a plug of some sorts or what...

 

looks like the bottom plug hole on tranny bell housing

 

Im gonna automatically assume that its important... does it hold in fluid or something? I saw where someone noted on a similar pic that it was a torque converter drain plug

 

No it keeps dirt and such out. It's an access hole for draining your torque convertor - I'm assuming that's an automatic. Either way you should get one and cover up the hole.

 

Cool, thank you for the advice.. very helpful

 

You can tell that there has been fluid leaking for a long time. It most likely deteriorated the original rubber plug.