On many Wordpress instances, WP-Cron, a peice of Wordpress software designed to run scripts at regular intervals, does not work as expected. Thank Me Later will automatically detect when WP-Cron isn't working, and revert to 'Legacy Mode' which introduces work-around measures to replicate (to a certain extent) the behaviour of WP-Cron.
In most cases, it is not worth trying to fix this issue since Thank Me Later will work near-perfect with 'Legacy Mode'. Certainly, for inexperienced users, the advice would be to stay away. If, however, you wish to fix WP-Cron, you can follow the advice in this thread.
Please contribute any fixes you have to this thread.
Note: If WP-Cron isn't working, it may take up to 2 hours from install for Thank Me Later to send any e-mails, regardless of the message configuration.