… it’s helpful to understand one of the basic mechanicals of reading: saccades. Instead of moving smoothly across the page when we read, our eyes actually make discrete jumps between words, fixating on one word for a short period of time before making a ballistic movement to another one. We call these movements saccades.

While we read, our eyes move unpredictably from word to word, rather than in a linear fashion.

via Typekit.