An array is a rectangular arrangement of objects in rows and columns. Arrays visualize groups of equal size and reveal that rows and columns can be switched (commutative property of multiplication).
You already know about equal groups — the idea that you can count faster by organizing objects into groups of the same size. An array is simply a way of organizing equal groups so they are easy to see and count at a glance. Instead of arranging six apples in a random pile, you put them in two rows of three. Or three rows of two. Now you can see the groups without counting each apple one by one.
A row goes across (left to right), and a column goes up and down. When you put rows and columns together in a rectangle, you have an array. If you have 3 rows with 4 objects in each row, you can count the total by adding: 4 + 4 + 4. That's three equal groups of four. Arrays make the equal groups you already know about easy to see because everything lines up neatly.
Here is something surprising about arrays: it doesn't matter which direction you count. If you have a 3-by-4 array (3 rows, 4 in each row), you can also look at it as 4 columns with 3 in each column. Either way, you get 12 objects. This works for every array — flipping the rows and columns never changes the total. That's a big mathematical idea hiding in a simple rectangular picture.
Arrays will help you later when you start learning multiplication, because "3 rows of 4" is exactly what 3 × 4 means. Every multiplication problem can be drawn as an array. For now, the most important thing to practice is seeing an array and being able to describe it two ways: by its rows and by its columns, and noticing that both descriptions give the same total.