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.A task ID usually looks something like f47ac10b-58cc-4372-a567-0e02b2c3d479, which is great for ensuring task ID uniqueness, but not so great for setting up a third-party integration with Habitica. To make this easier, you can set a task alias property under the advanced options when editing a task.
The task alias must contain only letters, numbers, dashes, and underscores. Providing any other character for the task alias will cause the save action to fail. To prevent conflicts with task IDs, a task alias may not have the same format as a User ID. In addition, task aliases must be unique among your tasks. Finally, a task alias may only be applied to a task owned by a player, meaning that challenge creators cannot apply task aliases to their challenges, but a player can add a task alias to their copy of the challenge task.