Demographic targeting is used to narrow down your audience to the right shoppers. Demographics are used in combination with your interest targeting to build the perfect audience.
You can use age and gender to do so. If you sell men's hair loss products, you might select Men between the ages of 40-65+. This way, your products reach people who are most likely to buy them. Demographic targets will limit who sees your ads, so be careful when modifying these settings.
If you're unsure of your demographic targets, it's better to select none and learn over time what they may be. Selecting the wrong demographic targets may prevent your ads from showing to the right people.