Why is a document locked?

A document can be “Locked” when you try to open it from the dashboard, this can happen for a few reasons:

Being processed by another user (In Progress):
If another user is already working on a document, other users won’t be able to open it. This is to prevent multiple people working with the same document and conflicting data.
If a document is already being processed by someone, in the dashboard it will be shown as:

When clicked to be processed, it will display as following: