ChannelReserve is the amount of BTC that the receiving party MUST maintain a balance above at all times. This is a safety mechanism to ensure that both sides always have skin in the game during the channel's lifetime.
The channel reserve is specified by the peer's channel_reserve_satoshis: 1% of the channel total is suggested. Each side of a channel maintains this reserve so it always has something to lose if it were to try to broadcast an old, revoked commitment transaction. Initially, this reserve may not be met, as only one side has funds; but the protocol ensures that there is always progress toward meeting this reserve, and once met, it is maintained.