What part of the toilet is this, and how to I fix the leak?

Hello,My toilet constantly drips onto the floor. I've had a google but I don't know what this part of the toilet is called so nothing is coming up that applies.​Here's a video (https://ift.tt/2Xn6UDI ideas? Even if i knew the terms that would help.​Thank you!​EDIT: Thanks everyone. I think i'm going to attempt to fix this myself! via /r/DIY https://ift.tt/3brEDUC