The symbol cc stands for cubic centimeter, while the symbol ci stands for cubic inches.
Both are units of volume and you can convert between them on the Volume conversion page. Another common unit of volume for engine size is the liter.
You cant directly convert volume to horsepower But in your case, you have a 15 horsepower Briggs and Stratton engine, you'll just need to know the year it was made, then contact the Briggs and Stratton company and they will be able to tell you. Possibly a dealer could help too, maybe even their website will have that information.