I have a custom object where I have this two Fields connectionreceivedid and connectionsentid where i can find this field while querying from workbench and when try to go Custom Field and Relationships this fields are not existing on that object.How come it showing in Workbench what is the use of this Fields.
These fields are available when in your ORG Salesforce to Salesforce is enabled.
ConnectionSentId: ID of the PartnerNetworkConnection that you shared this record with. This field is available if you enabled Salesforce to Salesforce. This field is supported using API versions earlier than 15.0. In all other API versions, this field’s value is null. You can use the new PartnerNetworkRecordConnection object to forward records to connections.
ConnectionReceivedId: ID of the PartnerNetworkConnection that shared this record with your organization. This field is available if you enabled Salesforce to Salesforce.
Every object contains many standard fields which are sometime system fields or fields that get unlocked when you enabled some features. These fields are not necessarily need to show on UI because they have some internal purpose to solve. On view salesforce only shows most relevant fields. You can use describe calls to get list of all fields like workbench.