In simpler terms, it's an indexing protocol, which indexes data on ethereum. Anyone can build an api called subgraph to query data on the network, and anyone else can use this api for their own dapp or other purposes. Uniswap, Aave and lot of others have implemented and deployed subgraphs. You can check all the subgraphs at https://thegraph.com/explorer/

The network has two main roles

Indexers : They run the graph nodes, process the queries and stake grt.

Curators : These people kinda tell indexers, what to index, by staking on a particular subgraph, if it gets queried more, they will get corresponding rewards.

The registration for testnet curator program is still ongoing, if you want to contribute.