The Task Alias field appears in the Advanced Options of a task. It is intended for use with some third-party integrations. If you don't use those integrations, you can ignore the field; adding a task alias will have no effect within Habitica. Some third-party integrations will ask for a task ID or alias for use with the integration. Often this is to score a task for some action accomplished, like scoring down a Habit every time you visit a time-wasting website or marking a Daily as completed for performing some action.