You get Seals from Epic or Legendary chests at a 10% chance. There are 2 types of seals. Legendary Seals and Mythic Seals. These seals allow you to upgrade equipment to Legendary or Mythic rarity. To upgrade to Legendary rarity, you will need a level 6 epic equipment + Legendary Seal for that specific equipment. For Mythic, you need a level 9 legendary equipment + Mythic Seal for that specific equipment.
Just because you get a seal, doesn’t mean you have to actually use it. Most of the time you get a seal for a tier 1 equipment. You can ignore them. Not only is there a good chance that the hero will be replaced one day, so you waste void crystals by making the equipment level 6. But also tier 2+ equipment is so much more important, you will have to raise all of them to level 6 before it’s more cost efficient to make a tier 1 equipment legendary.