Course allocation is like choosing which toys to play with at recess or which snacks to have for lunch. Just like how you have a limited amount of time to play at recess and a limited amount of food for lunch, there is a limited amount of space in each class and a limited number of seats available for each student.
So, before the school year starts, the teachers and administrators have to figure out how to divide up all the students into their different classes. They look at things like your grade level, your interests, and what you need to learn in order to decide which class would be best for you.
Once they have an idea of how many students will be in each class, they have to figure out which classroom and which teacher will go with each group of students. Sometimes they have to move the teachers around to make sure that every class is covered.
After all the planning and organizing is done, the school will give you a schedule that tells you which classes you will be taking and when. This is called your course allocation. Just like how you can't have recess all day or eat snacks instead of lunch, you can't always choose which classes you want to take. But, the teachers and administrators try to give you the best possible schedule based on what they think will help you learn and grow.