Have you heard of families? Well, in the Philippines, families are very important and they have their own special names for each member.
For example, your mom or dad's siblings are called "aunts" and "uncles". And if they have children, those children are called "cousins".
When you get married, your spouse's family becomes important too! Your spouse's siblings are also called "aunts" and "uncles" even if they are not biologically related to you. And if your spouse's parents have other children, those children are your "brothers-in-law" or "sisters-in-law".
It's kind of like having a really big family with lots of different ways of being related to each other. And in the Philippines, the importance of family and these special kinship terms are very much a part of the culture.