Spacemesh is a decentralized platform based on a novel blockmesh, rather than blockchain, structure. Under the hood, Spacemesh uses a novel Proof of Space protocol, structured by a mesh (layered directed acyclic graph).
Users who want to run the Spacemesh protocol need to set aside some of their disk space. Spacemesh, like other blockchain systems, requires users to prove that a scarce resource is committed to the system. This is needed to ensure the security of the system. In Spacemesh, this resource is disk space, something that millions of people around the world have access to on their home PCs, which means that joining Spacemesh can be basically costless for the home user. Using disk space makes the Spacemesh protocol ASIC-resistant and consumes significantly less energy compared to blockchain systems based on Proof of Work.
Dive in to Spacemesh by clicking the numbered components in the navigation panel. If you would like to know more, you can always dive into the protocol paper, our github (which includes a local Testnet where you can see everything being described here in action) or the FAQ page - or simply ask your question directly on twitter, gitter, or telegram.