Can I lose fat (go down in body fat %) while trying to gain alot of muscle (maybe using a weight gainer????)?

I am so confused about the concept of putting on muscle and losing fat... some people say you can do both at the same time (put on muscle and lose fat) but I have also been told that they are conflicting terms and you can only do one of the two as one person told me putting on muscle requires you to eat more than 2500 calories a day... (recommended daily allowance) but losing fat requires you to eat less than 2500 calories a say... is it possible to do both??.... I have already come down from 105 kgs to 88kgs and I am 6'3 so I am not overweight but my body fat percentage is 18% and I would like to get it to about 10%... can I do this while lifting weights and using a weight gainer to put on muscle at the same time and if so how???? Help me clear this up once and for all!!!!!!!!!!