Cut an eighth in half and you get a sixteenth note. The standard count for a beat full of sixteenths is "1 e & a" — one syllable per sixteenth, evenly spaced. The e and the a are the two new pulses that didn't exist when you were just counting eighths.
Sixteenth notes are where funk, R&B, and most modern drumming live. Even when not every sixteenth is played, the grid is what your time is locked to.
Making the e and a Real
At first the e and a are just sounds your mouth makes; the goal is for each to become a distinct place you can aim a stroke at. A good drill on top of the exercises: play straight 16ths and accent only the e for a bar, then only the a. If the accents wander, the positions aren't real yet — drop to ♩=60 and rebuild.
The tell-tale failure is smearing: e-&-a blurring into a fast triple instead of three even positions. Smearing means the tempo is ahead of your counting, never the reverse. Read Ex 4 by counting it, not by memorizing its shape; placing a note on any syllable of "1 e & a" on demand is the skill this lesson exists to build.