Steg, also known as Steg The Slug, is a 1992 game developed by Big Red Software and published by Codemasters for the Commodore Amiga, Amstrad CPC, Atari ST, Commodore 64, MS-DOS and ZX Spectrum. More information at Wikipedia.

A trainer offers one or more cheats like unlimited lives, invincibility, more ammo etc. Because this is done during runtime a trainer is not to be confused with a savegame editor.