For Q1:
You might have seen in many analog filter design books, that a RC combination can act as both low pass (see attached fig.1) and also as high pass (see attached fig.2).
For the lowpass configuration, a resistor allows low frequencies and the capacitor does not allow low frequencies (Xc = 1/wC), since at low frequencies high impedance is presented by the capacitor, so fig.1 allows only low frequencies. So only low frequency signals appear at the output.And for high frequencies, capacitor acts as short (low impedance) and they are grounded.
For the highpass configuration, capacitor allows high frequencies(Xc = 1/wC), as low impedance is presented by the capacitor at high frequencies, and high frequency signals appear across the resistor and at the output. And for low frequencies, capacitor acts as open (high impedance) and hence only high frequencies pass through the configuration shown in fig.2
For Q2:
Capacitors are used across power rails to reject the high frequency noise from the external sources, in other words to short the high frequency noise signals to ground. So if your capacitance value is high, the impedance is low (<10 ohm), and they acts as short to ground. Generally
0.01uF capacitors are used across power rails.