Cosmos is a project built around the idea of connecting many independent blockchains so they can communicate and share data. Rather than one chain doing everything, Cosmos envisions a network of specialised chains working together, which is why it is often described as the internet of blockchains.
A key part of this is a set of tools that make it easier for developers to build their own blockchains, plus a communication standard that lets those chains exchange information. ATOM is the token associated with the central hub of the Cosmos network, used for functions like securing it through staking and governance.
This entry explains what Cosmos is and the problem it aims to solve. It is educational and describes the technology and its purpose. It is not a view on the token as an investment, and nothing here suggests buying or selling any asset.