In my mind, it's one of the easiest ways to get around London's amazing network of transportation options.
It's a small, credit card sized card you can use to tap in or tap out of public transport such as the London Underground.
It's a £5 deposit for the card itself and then you can top it up with change at some machines or with a debit or credit amount at many others and in-person counters. You can either return the card and have the deposit amount and any outstanding balance returned, or you can keep the card for a future visit / as a souvenir.
The convenience is in being able to just swipe a contact card like local commuters do, which means there's a daily limit to how much you'll be charged; just remember that London's Tube system is metered by Zone and that your upper limit can change depending not on how many trips you take in a single day, but by the Zones in which you use your Oyster Card.