Flossing is important to get food debris out between teeth and to prevent cavities between teeth. Food debris can be trapped underneath the contact points between teeth, and it can stick to teeth forming a particle called plagues. Once the plaque builds up for some time, it hardens into a calculus, which once it forms, you need professional dental cleaning to remove them. They are a collection of bacteria and cause gum disease and cavities. With a good toothbrushing, most of the plagues are removed but not all, therefore flossing is an essential part of complete dental care. At the beginning of flossing, the gum may be bleeding but over time, your gum should get healthy enough so it stops bleeding during flossing.

Proper flossing technique

Spend a minute or two for the flossing
Be gentle so it doesn’t damage your gum tissue
Put a floss through between teeth from the top surface of teeth but when you take it out pull it out to the side so it doesn’t damage any of your previous dental work, such as filling or crown. Once you put the floss through, gently push down toward gum tissue so it gives proper irritation to the gum tissue. With a proper irritation, gum gets healthier and eventually bone gets healthier.

What age do I start flossing?

Sooner the better. For most people, primary teeth (baby teeth) have space between teeth called “primate space” so they don’t contact next to each other, so flossing may not be absolutely needed but sooner you get into the habit of doing flossing is better for you, because it takes some time to get used of it.

References:
https://www.ada.org/en/member-center/oral-health-topics/floss
https://en.wikipedia.org/wiki/Dental_floss