Help me understand this circuit schematic
C4 is a decoupling capacitor, it removes noise. It works on the high-frequency noise. You should have small decoupling caps on all digital chips, close to them. See the data sheets for the parts for suggested values, often 0.1 uF to 0.01 uF.
A schematic shows connections. All it is saying is C2 and C3 are connected to ground. It doesn't say if that is a single wire or two.
C1 is also a decoupling capacitor. It works on lower frequency noise, for example that from a power supply. It complements C4. You should have this sort of part on all boards, but location isn't critical.


4 &amp; 5. That pull-up is required by the USB spec. That's how the computer/host knows if it is low-speed or full-speed USB. If it were the other speed, the pull-up would be on D+.