Overview

In the Lightning Network, a preimage is a data string that is passed into a hash function. Preimages are also referred to as 'secrets'. Possession of the relevant preimage allows users to claim HTLCs, which are a core process in the routing of payments between nodes.

References

[1] https://bitcoin.stackexchange.com/questions/48053/what-is-a-hash-pre-image-as-it-is-used-for-the-breach-remedy​