We read this out as "negative 15" or you could write "negative fifteen".
The negative indicates the temperature went down. It doesn't necessarily mean that the final temperature is -15 degrees.
For example, if it starts being 50 degrees F, and drops by 15 degrees, then 50+(-15) = 50-15 = 35 degrees is the temperature after that drop. It might help to use a vertical number line.
