The lock symbol 
can be seen in a document that is part of a binder.
As long as the document is locked, it means (among others) that:
- certain actions cannot be taken, such as changing the order of clauses, inserting new ones or removing clauses from the document; and
- any updates made to the ‘original’ document (i.e. the document outside of the binder) will also be automatically made to the document in the binder, making sure any updates to the template are reflected in the binder template as well.
Unlocking a document (by clicking the lock symbol) then has (among others) the following effects:
- clauses can be (re)moved and inserted freely; and
- importantly, the link between the original document and the document in the binder disappears, meaning that any changes made to the original document will not be reflected in the document in the binder.
In view of the fact that unlocking a document breaks this link between the document in the binder and the original document, careful consideration must be given prior to unlocking a document in a binder!
For more information, please consult the manual article on locked documents.
Clause hierarchies also have a lock symbol once they are inserted in a document. Check out this article on clause hierarchies for more information.