These are discounted (sometimes heavily discounted) tickets, but they can only be used on the specific train they're booked for. While they're good for saving money, they're tricky if you're trying to book tight or uncertain connections. If you miss your train, you're out of luck. Go for these only if you know yourself, and know you can be reasonably punctual.
Compare advance singles and return fares
Comparing the different fare types can be a bit dizzying and difficult, depending on the website you’re using to search, so it's worth making sure you know what kind of ticket you’re buying. Even though Advance Fares are marketed as the cheapest, Return Tickets are often a better deal, and even when they're not cheaper, they nearly always give you far more flexibility for about the same price.
Additionally, many routes have Same Day Return fares that are mere pounds (sometimes mere pence) more than comparable Single fares, so if you don’t need to stay overnight, make it a day-trip! A Same-Day Super Off-Peak Return on Thameslink from London to Brighton costs £10.50, compared to two advance singles, which can start at £17!