When the interval has passed (is this correct English?), the CAP will perform a background scan to check if the current frequency is optimal. It will reselect if there is a better frequency (and perform a DFS check from 1 to 10 minutes in addition befor activating) available.
It will choose a different frequency from which are available. This can be specified explicitly and depends on regulations.
Sure you want to have this checked every 10 - 15 minutes?
And from the documentation that you already have found:
Specifies when the interface should rescan channel availability and select the most appropriate one to use. Specifying interval will allow the system to select this interval dynamically and randomly. This helps to avoid a situation when many APs at the same time scan the network, select the same channel, and prefer to use it at the same time. reselect-interval uses a background scan.
The reselect process will choose the most suitable channel considering the number of networks in the channel, channel usage, and overlap with networks in adjacent channels. It can be used with a list of frequencies defined, or with frequency not set - using all supported frequencies.