C21 is a decoupling capacitor. It reduces the impedance at AC on the junction of R33 and R15 therefore establishing a stable voltage there. The amplifier looks like a summing amplifier so this reduces the “wiggle” on the DC offset input at DC and allows AC to float on that DC level. At DC the capacitor is effectively open.
You can model this by drawing the circuit twice. Once with the capacitor shorted (at DC) and once when it’s open (at AC) and think about each case independently.