Two notes per beat can be felt as even (straight 8ths: tee-tee, tee-tee) or swung (shuffle 8ths: tee-tuh, tee-tuh). The shuffle is the second one — every pair of 8ths is reshaped so the first note is roughly twice as long as the second. The simplest way to think about it: skip the middle note of an 8th-note triplet. Boom-(rest)-tuh. That gap is the shuffle.
Layered on top of a basic rock backbeat — kick on 1 and 3, snare on 2 and 4 — you get the shuffle feel: the rhythm of half the blues catalog, most country shuffles, the early rock-and-roll of the 1950s, and the gateway into medium-swing jazz. In notation we write 8th-note triplets with the middle note missing (a quarter note + an 8th, beamed as a triplet), or simply note "swing 8ths" above straight 8ths and trust the player to interpret it. The exercises here use the explicit triplet notation so you can see the time grid the shuffle is sitting on.
Why this matters: the shuffle is the single most useful gateway feel a beginner can own. Once it's in your body, the leap to a jazz ride pattern is smaller than it looks; the leap to a country two-step is shorter than that. It is, in many ways, where the next half of your education lives.