Unilateral force closure of a channel by a party.
A channel is in fact a shared wallet between two or more users. When you move money in a channel its and internal records, not on the bitcoin blockchain. When dispute occur, or when a channel no more can function well , one of the user can decide to call for channel closure and the last record within the channel will be recorded to the bitcoin blockchain.
Example.
Suppose I & U created a channel with 1 btc each.
So the channel now has 2 btc. During channel activity, you sent me a net 0.5 btc. So off chain, i now have 1.5 btc and you have 0.5 btc, but this was never recorded on the blockchain. Supposed i fought with you and decided i no longer needed a channel with you. So i force close the channel. The last record of balance between me and you will be broadcasted on the blockchain.
So i will now have 1.5 btc and you will have 0.5 btc
