Modern computers tend to have many gigabytes (i.e. billions of bytes) of RAM (memory). This vast memory can be easily accessed as a giant sequence of words or bytes. We refer to the content of words/bytes in memory using their (0-indexed) position in the sequence. Such a fixed-size sequence that can be indexed by position is called an array.
The best thing about an array in RAM is that we can trivially & very quickly inspect or change a word anywhere within it as long as we know its position. This makes arrays ideal for analysing or editing large quantities of identically structured data that have an obvious natural ordering.
We can sometimes exploit the fact that arrays are guaranteed to contain data of the same type in order to perform identical computations or manipulations on an entire array in parallel. This technique is known as SIMD (single instruction, multiple data).
Common examples of array data include statistical records, images (grids of pixels), and text corpora. Speaking of which, let's see how we represent text on computers.