What does the word "spell" mean here? "The level of holiday sales can spell the difference between profit and loss for many stores." Thanks.

Inside No.9 Last night I watched the BBC drama Inside No.9. I was impressed by the crazy imagination of the writers. Those stories are a little horrible but interesting and surprising. I love the third story TOM&GERRI most. It tells about a teacher, Tom, who dislikes his current job and dreams of being a good author like Charles Bukowski. One day Migg, a tramp, knocked at Tom's door and said he found Tom's wallet. Tom gave Migg some dollars for appreciation but still looked him down because of his dirty clothes and messy hair. However, after knowing that Migg knew Charles Bukowski, Tom became friendly to him. They drank together and became good friends. But Migg was cheating. He deleted Tom's phone messages ,hid Tom's letters, stole his keys and took away his money secretly. Finally Tom lost his job, used up his all money and lost his girlfriend, he became a tramp as what Migg was. But Migg shaved his beard, put on clean suit, found a good job just like what Tom was! Things have totally changed! Finally Tom's grilfriend, Gerri, came back and told Tom that Migg was just someone that he imagined, Migg was not a true man at all! Gerri also said that Tom must get rid of Migg and then his life could turn back normal. Several days later, Tom seemed much bette. Hi's colleaguea, Steve, came to see him and told out an amazing thing —— Gerri had died long time before! And when Steve stepped into the bathroom, he was shocked to find a dead man in the bathtub. Then Tom said, there is no one dead, he is just what I imagined... So Gerri is actually the imagined one rather than Migg! Migg is a true man and he was killed by Tom but Tom didn't realize that. What a tragedy! Have anyone watched this drama before? If you don't, I think it's worth a try. And if you have any other interesting dramas or movies to share, I'd be very happy. Thanks for reading. If there is anything wrong or not proper, please tell me. I want to know how native speaker express themselves.