An "oath of allegiance" is like making a promise to be loyal to someone or something. It's a special promise that people sometimes have to make when they become citizens of a country or when they start a new job.
Imagine if you were going to start a new school and you had to promise to be a good student and follow all the rules. That's kind of like an oath of allegiance!
When people make an oath of allegiance to their country, they promise to be loyal to their country and follow its laws. This means they will respect the leaders of their country, be a good citizen, and love their country.
Making an oath of allegiance is a way for people to show how much they care about the place they live and the people who live there with them. It's a very important promise that people take very seriously!