Pinning Content on IPFS
By default, content on IPFS is not pinned. That means when you add a file to IPFS, it will eventually stop being discoverable on the network.
This problem occurs because IPFS uses garbage collection to free disk space on your IPFS node by deleting data that is no longer needed. Garbage collection is a form of automatic resource management widely used in software development.
To mitigate this problem you must pin your content to your node to ensure it persists on IPFS. Your options for pinning content are:
You pin it locally to your own node. Note that if the content is only pinned to your local node, it must be online for peers to get that content.
You use a pinning service. Some pinning services require payment for their services, others offer a free storage allowance for new users.
参考：
https://curriculum.pl-launchpad.io/curriculum/ipfs/content-addressing/