If you remove a field from the Layout tab, you are only removing it from being seen on the plus tab.  You are not removing it from the database.  The data is still in the associated table and can be reported on in reports, exports, or datagrids.  You can also add it back to the same Layout or a different Layout at any point.

If you remove the field from the Field tab, and Commit Changes, the field will be permanently removed from the database and all the data in the field in all records will be lost. It is recommended to first remove the field only from the Layout.  After an appropriate amount of time has passed, and you are positive your organization will never need access to the data, you can remove it from the Field tab as well.