Amortisation usually appears after an acquisition, when the purchase price is allocated to intangible-assets such as brands and customer relationships. Those get written off over a set number of years; goodwill does not amortise and is instead tested for impairment.
Because most amortisation is an artefact of past deals rather than ongoing spending, many companies report an adjusted profit excluding it. That adjustment is one of the more defensible items in a non-gaap bridge, but it still reflects money that was genuinely paid.
Example: Northwind pays $180M for a battery maker and allocates $66M to technology with a six-year life. Amortisation is $11M a year, so GAAP net-income is $11M lower than the pre-deal business would suggest each year.
Related: depreciation, impairment