Cáscara is the timbalero's shell pattern: ten strokes over two bars, played on the sides of the drums (and on the cymbal or bell when the band opens up). This is the 2-3 form — Levine's fig. 20-14, the pattern he puts on the drum machine once you can hold a montuno against quarter notes.
How it sits on the clave: it is a filled-in clave. Every clave stroke is in it (2 and 3 in bar 1; 1, and-of-2 and 4 in bar 2), and the extra strokes fill the gaps on the beat and the and-of-3. The 3-2 form (Scivales Ex-115-3) is the same thing started from bar 2.
Because the DSL has one percussion layer per pattern, the clave layer here is the cáscara; the 2-3 son clave it implies is on the badge. The piano plays Levine's basic C montuno (fig. 20-12) and the bass a root-fifth tumbao, so you can hear the three parts lock.
Practice: mute the piano and tap the cáscara with your right hand while your left taps clave.