It's the sphincter muscle at the junction of the ileum (last portion of your small intestine) andthe colon, first portion of your large intestine. Its function is to allow digested food to pass from the small intestine to your large intestine. It acts as a barrier between the large and small intestine. It prevents the backflow of harmful toxins and bacteria overgrowth into the small intestine. It also prevents food from entering the large intestine before it's effectively digested.
For me, it's caused a real problem with SIBO ( small intestinal bacterial overgrowth) — the resection site (anastomosis) is wide open with no valve so there is no way to keep bacteria out of the small intestine. I do take an antibiotic daily callled Xifaxan which has made my daily life 100% better.
You might want to ask your dr about what the plan is regarding removal of the valve. I have to warn you, many GI's downplay the significance of the IV.