The file extension LCK is used by various programs for so-called Program Lock files. These files are used to restrict access to other files, such as a database, to one user at a time. Usually, this file will be automatically opened with the appropriate protected file, and then it will be locked so that if another user attempts to open the protected file, it will be denied.
Programs that use LCK files include Microsoft SharePoint Designer, Corel Paradox, Pegasus Mail, and Skype.
Of course these files are not compatible with each other and should neither be opened nor moved from their directory or deleted.