TODO is often used in code comments to indicate a task that needs to be done. IDEs often highlight them. Please do not use this for questions about creating todo list applications.
TODOs are source-code comments, stating that something still needs to be done in that particular section of the program.
Different syntaxes are used for different languages. A typical C-style example would be:
// TODO: Fix this
It has been debated whether todo comments are to be considered good or bad practice.
Most IDEs have the facility of highlighting TODOs and/or displaying them in a separate view.