why does my mouse have a lock symbol when trying to dimension?
This is a feature which is available when selecting two entities which could be dimensioned in more than one way, say a horizontal dimension or one at an angle etc. If you drag the dimension and its displaying as you want you can right click to 'lock' it to that dimension type.
There is a better explanation here: 2016 SOLIDWORKS Help - Locking Dimensions
To help you to lock the orientation of the dimension.
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