A "seal" is something that keeps something closed. There's actually a wide range of things that can be called "seals":

• When you install something like a window into a new home, you have to use a substance to seal the area around the window.
• If you go diving in the ocean, there's a seal around the area of your mask where it touches your skin and doesn't allow any water to get in.
• In the Middle Ages, kings used to use wax to seal their letters.
• There are seals on lots of products. The seals show when someone has opened the product.

"Breaking a seal" means opening up something that was closed and sealed. So, for example, if you open a sealed letter, you've "broken the seal".

There's also a funny meaning to "breaking the seal". You can read about it onUrban Dictionary.