Answer to the first and second questions:

A payment channel is a means of transaction outside the blockchain, in which two people commit funds to an address and pay each other by issuing payment commitments signed by the parties, avoiding having to wait for confirmations from the underlying blockchain. .

Answer to the third question, why is it difficult to open a channel?

The parties open a payment channel between themselves by sending funds in an initial transaction (Funding Transaction) to a 2-of-2 multi-signature address, which is managed by both parties and requires the signatures of both parties to generate new transactions. This first transaction is then propagated to the blockchain, leaving the channel effectively open.