Keeping one
Every result carries a save control. Use it and the flight joins the list under Saved, which is the shortlist to compare against rather than a booking of any kind — nothing is held or reserved by saving it.
The list is priced fresh
What gets kept is which flights you picked, and nothing about them. Prices and times are read again every time you open the page, so a saved flight always shows what it costs now rather than what it cost when you saved it.
That is the useful part. A shortlist that quietly showed last week's fares would be worse than no shortlist, because you would only find out at checkout.
It belongs to this browser
The list lives in this browser, like your currency choice. There are no accounts here, so it does not follow you to a phone, and clearing your browsing data clears it. Left alone, it keeps for a year — which is longer than your bookings last, since those go with the browsing session and not with the browser. See Ticket did not arrive.
When one disappears
Flights come and go, and this site regenerates its network. If a flight you saved is no longer sold it simply drops off the list rather than sitting there unbookable. Nothing else on the list is affected, and the fix is to search the route again — see Searching for flights.