The phrase gets attributed to Mark Twain ("eat a live frog first thing in the morning and nothing worse will happen to you the rest of the day"), though nobody's fully sure he said it that way. Either way the method is blunt: find the task you're most likely to avoid, and do it before anything else gets a chance to. Not the biggest task. Not necessarily the most important one either — that's a related but different idea, more on that below. Just the one you'd otherwise let slide to 4pm, then tomorrow, then next week.
It's worth separating this from picking your Most Important Task, because dread and impact don't always point at the same thing (the annoying phone call you're dodging might matter less than the report due Friday, but it's still the frog). This tool's Today list holds a handful of tasks for the day, and the MIT toggle lets you flag one as the one that has to happen first — use it on the frog, not just the biggest task, and it does the same job of putting one thing ahead of the queue. Just try it: pick tomorrow's frog tonight, toggle it, and eat it before the inbox opens.