There is a book of that name written by Brian Tracy. The book is 130 pages long but the main idea is very simple – deal with the task that you dread most as the first thing in the day.
It may be the least enjoyable, or the most time-consuming one that you have tendency to procrastinate and leave for the end of the day (which may eventually slip to tomorrow or even later). The problem is that it occupies your (subconscious) mind and takes mental activity away from other tasks. In some situations, I may prefer to clean my inbox and do quick tasks before starting to deal with the frog (e.g. when someone is stuck until they receive my response or when I need to wait for someone else’s input, in which case it is better to ask for it early), but generally I try to follow what I preach. That leads to two benefits – (i) I clean my mind and feel a sense of accomplishment early on in the day, and (ii) there is no risk that some other tasks fill most of the day.
And as I promised in the beginning of the blog, no need to write two pages, if the idea can be explained in one or two paragraphs – so this is it.