"Pin" is a nice word the IPFS team has come up with to designate the act of telling your node to store some content permanently and don't garbage-collect it. The idea is that you'll store everything you fetch and reroute this to others automatically, but every once in a while all content you have on your node that is not explicitly "pinned" will be erased, so you shouldn't worry about storing too much of other people's things, but also can contribute to keep alive content you like.
Pinning has a big problem, however: you can't know what you've pinned. Every pin you add is going to be saved on your computer, you won't be able to unpin stuff because you don't know what is what, in the end you'll be left with a disk full of pinned stuff and probably lose that disk or delete everything to open up space for other things after getting frustrated with the entire IPFS experiment.
Examine the incentives in this model: we're relying on sharing being made by people that do that unwillingly and unknowngly. Users spend electricity on nodes that are supposed to be always running and serving content to others. Links are only kept unbroken if someone decides to pin them, but since there's no order, the pins are doomed to be erased everywhere.