when you are selling to customers you need to do Goods issue.
When cyustomer returns (due to any reason) than you do goods receipt.
GI stands for Goods Issue, means when you do any sale, the goods will be issued from the warehouse to the customer. Here you will do a post goods issue of the delivery.
GR stands for Goods receipt, in sales perspective, when the customer returns the goods because of any reason, you need to receive the goods in to your storage location. Here you do a post goods receipt of the delivery.
To complete your finish product and send it to customer,you need some raw materials for example.
These raw materials procured externally,once the vendor delivers the raw material as per your requirements,you receive the goods and do goods receipt posting in SAP.Tcode is MIGO.This is from Materials Management point of view.
If you take Warehouse Managepoint of view.
Both are associated with movement types.
Reward points if my answer helps.