Open Ask on the unit and say you want it summarised. A small card appears with the unit's name and one button: Summarise. Tap it and the summary is written into the conversation.
It costs 18 credits. The card does not say so — your balance lives on Account → Usage, and the full price list is in What each thing costs.
It needs a unit
A summary is always of one unit. If you are in a conversation about the whole course, you will be told to open a unit first — there is nothing for it to summarise otherwise.
What it is good for
Revision, when you have read the unit once and want it back quickly.
Deciding where to start, when you have four units to get through and two evenings.
Checking what you took away, by reading the summary after the unit rather than instead of it.
What it is not good for
Replacing the reading before an exam. A summary is shorter because things were left out, and what gets left out is detail — which is often what a question turns on.
Where they are kept
Summaries stay in the conversation you made them in, and that conversation stays with the course. Opening one again costs nothing.