In English, a soul usually refers to the non-material essence of a human that survives after death, but that concept would be entirely foreign to the authors of the Old Testament. Biblically, people don't have a soul; they are a soul, or in this case "Nephesh"—a living, breathing, physical being. In the Shema, to love the Lord with your soul is to offer your entire being, with all its capabilities and limitations, in an effort to love God and to love your neighbor as yourself.
"Me'od." Interestingly, this is the only place in the Bible "Me'od" is translated as strength. Everywhere else, it means "very" or "much." It's an adverb that intensifies the meanings of other words. While it may sound funny, to love God with all your strength "Me'od" is to love him with all your "Muchness." It means to love God with everything you have, devoting every possibility, opportunity, and capacity to honor God and to love your neighbor as yourself.