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Bitcoin's base layer deliberately processes a limited number of transactions per block to stay decentralised and secure. That makes it robust but slow and relatively costly for small, frequent payments. The Lightning Network is the best-known attempt to fix that without weakening the base layer.

The scaling trade-off

A blockchain that everyone can verify on modest hardware cannot also handle unlimited transactions cheaply — increasing capacity tends to increase the cost of participating, which reduces decentralisation. This tension is often called the scalability trilemma (balancing decentralisation, security and scalability).

What Lightning is

Lightning is a layer-2 network built on top of Bitcoin. Two parties open a payment channel by committing funds in a base-layer transaction, then transact back and forth off-chain, instantly and almost for free, settling the final balance to the main chain only when they close the channel.

Channels connect into a network, so you can route a payment to someone you have no direct channel with, hopping through intermediaries.

Benefits and honest limitations

Benefits: near-instant settlement, very low fees, the ability to make tiny (micro) payments that would be uneconomic on-chain.
Limitations: funds must be locked in channels in advance, channel management can be complex, routing can fail if there is insufficient liquidity along a path, and a node generally needs to be online to receive. It is powerful but not frictionless, and adoption is still developing.