All music written in standard notation, on lead sheets, on charts, and in tablature shows the individual measures divided by vertical lines called bar lines, which are spaced fairly evenly across every line of music. In cases where tab is shown at the same time as standard notation — such as in this book — the bar lines may appear only on the staff and not be shown in the line of harmonica tab below.
You may recall that each measure in a given time signature has the same number of beats, as shown by the top number in the time signature. In order to keep track of where you are when reading music, you will need to count the measures as they go by. If you're playing in 4/4 time — by far the most common time in rock and blues music — each measure will have four beats, and will be counted 1-2-3-4.
There are many signature harmonica riffs in rock and roll that elevated the harmonica into the role of lead instrument, including John Lennon's opening of “Love Me Do,” Sugar Blue's introduction to the Rolling Stones' hit “Miss You,” the plaintive harp melody on Sting's song “Brand New Day,” and Neil Young's opening harmonica notes on “Heart of Gold.”
When you're just starting out playing over chord progressions, or when you're first learning a new song, you may find that you have to count all four beats of every measure to keep track of where you are in the progression. Once you're a little more familiar with the music you may find that simply tapping your foot on the 2 and 4 beats of each measure gives you enough grounding, and once you're very familiar with the music you probably won't have to count, or even think consciously about which beat you're on at all.