Abbreviation: A-M = "A-M switch" (Nikon)

The abbreviations means "A-M switch"

Explanation from Nikon

An element that locks the focus ring during autofocus while enabling behavior similar to a manual focus lens in manual focus mode with sufficient load when the focusing ring is rotated. The focus rings on the AF-S DX NIKKOR 18-55 mm f/3.5-5.6G VR, AF-S DX Zoom Nikkor ED 18-55 mm f/3.5-5.6G, and AF-S DX Zoom-Nikkor ED 18-55 mm f/3.5-5.6GII rotate during autofocus.