Being in the fitness business for a couple years; I've met a ton of people. One common myth I alway hear is "you need to do low weight and high reps for fat loss" and "low reps with heavier weight to gain weight." wrong. wrong. wrong. wrong. wrong!
Performing exercises with low weight and higher reps will help build up your muscular endurance. Muscular endurance comes in handy because it is the ability for your muscles to do a specific motion over a longer period of time without getting tired. Don't get me wrong, any type of exercise you are doing you are going to burn calories but that doesn't always mean fat loss. The only way to achieve fat loss is consuming less calories than you burn. Which brings me to my next point...
Exercises where you are doing low reps with a much heavier weight will help you build muscle. *oh no Sara I am a female and I heard that building muscle will make me look super huge like #RonnieColeman* Stop thinking that! Having muscle on your body is the KEY to continually burn calories around the clock. The more muscle you have the more you burn by doing abso-lutely-freaking-nothing.
Keep in mind though, that while still consuming excess calories even with the right training plan, you will not drop the unwanted fat. Nutrition and training go hand and hand. Its nearly impossible to achieve your goals doing one without the other. Thats where you hear a lot of fitness people talk about a "bulking" and "cutting" season.
Bulking season is where you eat in a surplus to gain as much muscle as you can. (you will put on some unwanted fluff) The key is to still eat smart. Cutting season is when you eat in a slight deficit to make sure you maintain all of the muscle that you put your blood sweat and tears into; while also decreasing your body fat percentage and getting "lean" or as some people may refer to it as "toned."
I hope this helps you get a better understanding of what these two different training styles can do and that muscle is your ''BEST FRIEND!''