If you delete a custom field it will remove the field from the item record, customer, purchase order, etc. However, if the field was deleted accidentally, you do have the ability to quickly add the field back in. To do this, you would need to create the field again by going to Settings>Custom Fields>New and create the field with the exact same name as the deleted field. Once you do this the field should repopulate with the same data.
Note: The only time you cannot delete a custom field is if that field has been used for grid creation. This is because the fields you use to generate new items on a grid determine what the item description will be.
Curious about how to create grids? Check it out here.