Short answer: The column is marked as ‘deleted’ and will stop being visible/usable.
But, most importantly – The record/table size will remain unchanged.
A metadata operation
Dropping a column is a metadata/logical operation, not a physical one. This means that the data isn’t removed/overwritten by this action. As Paul Randal mentions here:
« the cost of that will be deferred for the inserters and not for the deleters ».
Does that mean that the content is still visible?
Sort of. If you try to query the table, because the metadata of the table no longer knows of its existence, you can’t