Scanning is the process of taking a physical document and converting it into digital form.
An example is when a user wants to have a digital copy of their tax return or shopping receipts so that they can post it up on reddit for the karma shower and do not want to do the mainstream thing of taking a photo of it with their thumb in shot.
Most scanners will import a document as an image format. Certain applications can perform optical character recognition to turn the document into an editable form.
For computer hard disk checkups, see scan-disk
For network or host port scanning, see port-scanning