I suggest you to read the replies of members above your post, L2 means layer 2 networks that are built on the first layer or main layer of the coin we are talking about as in terms of BTC the main layer is BTC network but if we talk about l2 network they are built on the layer of BTC network and require us to pay the lesser fee and some are more secure, more compatible, faster, there are different versions of L2 like a taproot, Lighting network, Liquid network, and now we are seeing many apps being built of BTC network.

These apps are leveraging the L2 networks built for BTC on its main layer. To understand it in short, its like a shortcut way to perform the task using the main way. Or in another way, BTC is the main road, while the L2 are the fast lane where the tx are cost effective and fast.