As you can see in this picture lifted from an eBay listing, this is a container of Roseart's Bub'l Gel brand of toy slime. I remember seeing the commercial for it often in 2001 as it competed against the Nickelodeon Goooze that had come out around the same time, produced by Flying Colors Toys.
As the package says, you can stretch it, blow it into a bubble, and bounce it just as you could with Goooze. Unlike the Goooze, the Bub'l Gel package gave you a straw to use with it to blow bubbles, and each container came with a secondary toy inside the slime itself, such as a scooter, a rubber alien or a rubber dinosaur.
I post this as I used to often see this in stores back in middle school and I regrettably wrote it off as an inferior product, not appreciating the value you were getting with it.