Arrays are a basic structure to store data. They are like a sock drawer that lets you store and retrieve socks. An array will only let you store the same kind of things. They are super efficient at finding things at random, but no so great at adding or deleting things. In your computer, arrays are stored in a continuous chunk of memory.
Combining arrays together lets you model 1, 2 or 3 dimensional data. 1 dimension could be a list of letters forming a word, 2 dimensions an image and 3 dimensions a voxel!