To deal with a difficult or unappealing task first, rather than procrastinating and putting it off.
Apparently inspired by a quotation from Mark Twain, who said “If it's your job to eat a frog, it's best to do it first thing in the morning. And If it's your job to eat two frogs, it's best to eat the biggest one first.” Modern use of the phrase was possibly coined by motivational speaker and author, Brian Tracy, in his 2001 book Eat That Frog!: 21 Great Ways to Stop Procrastinating and Get More Done in Less Time.
The Quote Investigator website suggest that the link to Twain is a misattribution, instead citing French writer Nicholas Chamfort as the originator in the 18th Century.