A "shoo-in" is someone sure to win an election, contest, etc.  His/her popularity, abilities, experience, etc. are so great that it appears clear to the speaker that nobody else has a chance.  "Shoo", by the way, is the sound we make to frighten a small animal, to make the animal move (leave the house, enter the house, stay away from the meat on the picnic table, etc.).  You can use it for flies, dogs, cats, mothers-in-law, etc.

"Frank has such a wonderful personality and so many friends at this school that he's a shoo-in for class president.  And just look at the great job he did as vice president last year.  He simply has no competition."

"Shoo!  Don't even think about eating that piece of meat, you ugly, mangy dog!  Get away!  Shoo!  Don't look at me like that!  Get!  Movie!  Shoo!"