The Object type is essentially a Type which can store any other Type instance or it can be null.
It can be used as a "transporter" which you can use as generic storage and then later cast it back to its intended Type. This can mostly be seen when using the Box and Unbox nodes.
Other nodes that use this type frequently are the ToString (Object) node and Format String node, as these need a generic type like an object to convert into a string of information.