Bitcoin has a low transaction per second (TPS) limit, around 5-7 TPS. The Lightning Network is one of the proposals to overcome this limit. According to its creators, it can scale almost endlessly becaus it is only limited by how fast you can sign transactions and send them to your counterparty. On the other had, the Bitcoin blockchain will remain at 7 TPS as long as the blocksize remains the same.
This video explains how Lightning works.