It is the bridge between the blockchain and information from the external world.
Blockchains are designed to be very good at verifying the authenticity of data within the network. They are not, however, in and of themselves capable of knowing if that data is “true” or not. If you want to incentivize a friend with SWEAT to walk 5km a day, the blockchain needs to be reliably fed that friend’s step count information from an external source. That source is known as an oracle, the Movement Oracle in this case.