Even numbers (2, 4, 6, 8...) can be split into two equal groups; odd numbers (1, 3, 5, 7...) cannot. Recognizing this property helps with number classification and patterns.
You already know how to skip-count by 2s: 2, 4, 6, 8, 10... That counting pattern is exactly the list of even numbers. Every number you land on when you skip-count by 2s is even. The numbers you skip over — 1, 3, 5, 7, 9... — are odd numbers. So the two lists take turns: odd, even, odd, even, going up forever.
Here's the best way to understand what "even" really means: if you have an even number of things, you can split them into two perfectly equal groups with nothing left over. 8 apples split into two groups of 4 — no leftovers. 6 split into two groups of 3 — no leftovers. But try it with 7: you get two groups of 3 and one apple left over. That leftover is what makes 7 odd. Even means pairs with no leftovers; odd means one is always left alone.
You can use this idea to check any number. Take 10 objects and try pairing them up — do they all pair? Yes, 10 is even. Take 9 — one ends up without a partner. 9 is odd. Another quick trick: look at the last digit of any number. If it ends in 0, 2, 4, 6, or 8, the number is even. If it ends in 1, 3, 5, 7, or 9, it's odd. That's why 100 is even and 101 is odd, even though they're big numbers.
Even and odd numbers show up in patterns everywhere. When you add two even numbers, you always get an even number. When you add two odd numbers, you also get an even number. But when you add one even and one odd, you get an odd number. You don't have to memorize these rules — you can figure them out from the "leftover" idea. Two odd numbers each have one leftover, and those two leftovers pair up, leaving nothing extra. That's why odd + odd = even. Noticing patterns like this is what mathematics is really about.