The full saying is usually given nowadays as: "What you lose on the roundabouts you gain on the swings" or vice versa. This was a reference, in an old poem, to a showground-owner's claim. In other words, on some days the kids would pay to pile onto the swings and provide his income, though the roundabouts were neglected. Obviously, it would be vice versa on other days. Today, of course, it applies to any such 50/50 or up/down situation.
The poem �Roundabouts and Swings' is by Patrick Chalmers and here are the appropriate lines, after the poet asks the fairground-man what his work is like:
"Said he 'the job's the very spit of what it always were,
'It's bread and bacon mostly when the dog don't catch a hare,
'But looking at it broad, and while it ain't no merchant kings,
'What's lost upon the roundabouts, we pulls up on the swings."