Imagine you're a medieval villager. You have access to a common - a fertile pasture that is owned by all the villagers together. The common can support 100 sheep - any more and the pasture would turn into a muddy wasteland. So the 20 households in the village each graze 5 sheep on the common, including yours.
One day you have a brilliant idea. If you got another sheep, that would only add 1% more burden on the common. That's not enough to turn it into muddy wasteland, surely? But you would get a benefit of 20% more sheep for your household. That's a big benefit for such a tiny disadvantage to the common. You can't resist it, and the next day you buy another sheep.
But what happens when your neighbours notice? They want an extra sheep, too. They also do the maths and realise they can have a huge extra benefit to their household whilst only placing a small burden on the common. Soon everyone has 6 sheep each, and now the common is supporting 120 sheep rather than the 100 it can sustainably manage. In fact some people think they could probably keep 7 sheep on the common, and maybe even a cow.
Within a year all the grass on the common is grazed away. The feet of the sheep (and the cows and, for some reason, a kangaroo) poach the earth, churning it up and preventing the grass from re-growing. When the rain falls the common becomes a mudbath. When the sun shines the mud turns to dust. When the wind blows the dust blows away. Where once there was a fertile pasture capable of supporting 100 sheep and 20 families, now there is a barren dustbowl.
This little parable is often used to explain why nobody takes care of things that nobody owns. Our atmosphere, our seas, our fish stocks, old growth forest, the climate - each of us can get a big benefit whilst causing only a small amount of additional burden on these things, these commons. But 6.6 billion people on the Earth are placing too much burden on them, and if we don't stop we could end up with a dustbowl planet, incapable of supporting human life.
Economists argue about the solution to the tragedy of the commons. Some claim that everything should be owned privately - if the common in the parable was owned by a landowner who leased it to the villagers then they would not be able to overgraze it. But how can anyone own the atmosphere or the climate? Other people want governments to restrict people from over-exploiting the commons by enacting laws against pollution, overfishing and so on. My preferred solution is for individuals to take personal responsibility and control their own behaviour for the common good, for example by cutting their carbon footprint, buying local food even if it is more expensive, and avoiding over-consumption in general. But I always was hopelessly idealistic.