What is a lip tie

Someone please explain a lip tie to me....

I asked my doc if my LO had a lip tie and she asked me back what is a lip tie!!!!!!!

Is that even a thing??? Doc said there's tongue tie but what is lip tie.... I was taken aback as I had seen ppl mention lip ties on glow and wanted to know if my LO had a lip tie as her upper lip doesn't really open out as much as her power up while latching on the breast to feed.... someone pls explain...and preferably with pics so that I can better understand this...