1. Within or administered by means of a vein; such as, an injection.
2. Existing within or entering through a vein or veins.

Intravenous comes from intra-, "within" or "via", and venous, "of the veins". Often in hospitals patients will be given water or nutrients from intravenous drips from plastic bags of liquid connected to a tube going into a person's arm.

It's a way to get needed nutrition and fluid into a person's body without waiting for the digestive system to process it.