ID and OD is as what dawp says.
You tend to see that mostly in tubing.
The difference between the OD and ID = tubing thickness.
Usually the thicker the tubing, the greater bend radius your going to get b4 the tubing collapses under torque.
Barbs will rarely have ID and OD on them.
The Thread Size usually represents the opening hole and barb threading.
G1/4 G3/8 .. so on.
They will tell you what tubing they can fit.
So if the barb says 1/2, its 1/2 inch tubing for the ID.
That means u would look for 1/2 inch ID tubing, which comes as dawp once again said, in 5/8 and 3/4.
the OD is important when your doing compression fittings tho.
You need to match OD with compression fittings or it wont work.