“Boo” means beautiful; a term of affection which originally was derived from the French word “beau”. Today, “Boo” is commonly used more as a popular slang word which references a cute nickname for a boyfriend or girlfriend. Boo is also used while flirting, or giving someone the idea that you admire or feel comfortable being around.
When I hear someone say “I love my “Boo”, I am thinking of a relationship that has moved toward a closer and more serious friendship. I would expect that these two individuals have begun developing their thoughts toward a love relationship and their discussions are about starting a future together. Their thoughts of a future should bring them closer in affection, closer to finding out more about each other, closer to what kind of life they would live, and hopefully closer to a bond of marriage.