Complex sentence: Each Lamport key can only be used to sign a single message, but combined with hash trees they can be used for many messages, becoming a fairly efficient digital signature scheme known as a Merkle signature scheme.
Split into simpler sentences: Each Lamport key can only be used to sign a single message. But combined with hash trees they can be used for many messages and then become a fairly efficient digital signature scheme, known as a Merkle signature scheme.