This is amazingly cool, cheap, white or yellow stuff. It usually comes on a roll and can be used for connecting different sized bits of stuff together when one fits inside the other. It could even be used for making a square peg fit into a round hole with a good seal, if you had enough. It doesn't have any glue or anything like that but it's slightly sticky.
To use, you just run it around the surface of the smaller diameter item that will be the inner object. Then, you shove the inner object into the larger object. If you have the right amount of tape, you'll have an airtight seal between the two objects. Very handy for plumbing endevours such as building a still. Cool, eh?