lightningd automatically tracks channels internally and will make sure to reconnect to a peer if it has a channel open with it. However, it only tracks the channel itself, it does not re-open a channel that was closed.

The persistent channels plugin allows you to describe a number of channels you'd like to have open at any time and the plugin will attempt to maintain that state. The plugin keeps a list of desired channels that should be opened and operational and will check every 30 seconds if it needs to open a new channel, or re-open a channel that is currently being closed.