Object store is a storage container which stores key-values information. Object Store can be persistent or non-persistent. Persistent OS are those which will not lose any information (key-value) in case of application restart where in non-persistent (key-value) information is lost.
In Cloudhub/On-premise Object Store is used to store data temporarily and that to without using the external storage like DB or File.
More information – https://docs.mulesoft.com/connectors/object-store/object-store-connector