From IFWiki

Initially, a trademarked name for the hint booklets published by Infocom; now a generic term for a way of organizing a series of increasingly explicit hits that do not unnecessarily destroy the challenge of solving a puzzle.

Each of the Infocom Invisicules books included a series of questions that the player might ask about a particular puzzle (e.g. "How do I find the cave entrance?"), and a series of increasingly specific answers written in invisible ink. The player used a special marker to reveal progressively more specific answers (e.g., first "Caves are created by water." then "Water runs downhill.", and then "Walk downstream."). To prevent players from finding out too much about the plot just by perusing the questions, Invisiclues frequently included humorous fake spoilers.

While the printing method is not practical for non-commercial releases, some people have created Invisiclues-style hints for their and other people's games.