We'll tell you what the main idea is. We use numbers to solve problems. A simple problem such as "If Ellen has 3 walls on which to watch the paint dry, and she watches 1 of them, how many walls are left to watch?" has a simple answer. As problems get more interesting, as surely they must, so do the numbers we need in order to solve them.
We'll go over each and every one of the main types of numbers you'll need in an algebra class. We'll cover natural numbers, whole numbers, integers, rational numbers, irrational numbers, and real numbers. We're going to skip over wrong numbers, because you can just hang up on them anyway.
We'll also talk about different ways to express the numbers we already do know, usually in the form of fractions, decimals, and percents. Because it can get boring to always write "1" as "1," and sometimes we just need to write it as 432,588 ÷ 432,588 to keep life interesting. Don't put that as the number of dependents on your 1040, though. The IRS tends to find such shenanigans unamusing. Lighten up, IRS.