Lock Form action locks the item window. If the action is activated, buttons and fields in the item window are locked.
It causes that the item cannot be edited or even deleted. This actions disables also all shortcuts that are associated with buttons. E.g. when you use the Lock Form action on a document, you won't be able to save revised document to the locked one.
For example: Lock Form can be used in the case when employee requested leave. Once supervisor approves his request, the request cannot be edited anymore. This action can be used also for a time sheet, document, etc.
Action definition has no parameters. The lock will be performed in the item window that is related with the workflow. If the workflow with the Lock Form action is defined for leave, the Leave window will be locked.