Dear Sally,
Check in and Check out are two important operations in DMS. Initially when you want to submit the document say a drawing into SAP Content server, you will do check in operation and yes check in means locked. Check out is Lock Open,
Simple Ex. Designers A,B, C are sitting in different locations/different countries and they are all working colloborativiely. Person A will complete his taks, check in. Person B will check out and work on his part and Check in. Then finally Person C will check out and do his part and check in. This is how one can use Checkin & Check out process in SAP. Hope this helps.
BR,
SM