Siacoin is a service that provides decentralized file storage - allowing you to store files like photos, videos, and documents on the cloud. This is very similar to dropbox or google cloud, with one major difference, files are stored on a decentralized network of computers.
This release has several significant changes. Full release notes will be available when v1.3.0 is fully released, but here are some highlights:
Prior to this release, the transaction pool was capped to a very small size (due to performance bottlenecks), and if transaction pools started to fill up, transactions could stop propagating across the network. As of this release, the transaction pool is larger, has a proper fee market, and has several other modifications that mean it will run better. The ultimate result here is that when you send money on the network, you can expect it to get into a block, even during times of high traffic