You may have a lump on the back of your finger, just near the joint closest to your fingernail. This lump may change in size, and be really irritating when you accidentally knock it against a wall, or when you are putting your hand into your bag or pocket. You may have popped the lump, only to get this clear, jelly-like stuff come out. The lump may have then come back again later. Sometimes the lump is associated with your nail changing shape in time.
This is most likely a mucous cyst. It is a pocket of thick fluid that originates from the joint of your finger, popping out through the lining of the joint. Popping it, or getting it aspirated (“sucked out with a needle”) does not usually resolve it completely, and if often recurs. Because of its connection to the joint, there is a risk with every time it pops, that bacteria from the skin surface can infect the joint itself, and cause a joint infection that can cost you the joint itself.
These are best assessed by a hand surgeon, who can discuss how these develop, and what your options are. In quite a few cases, these are cosmetic, and do not need treatment. However, if your skin looks translucent or threatened, or if it is quite irritating, surgery may be your best option. Consider booking an appointment with a hand surgeon to get this addressed.