Figure 3.2 shows one possible low-pass filter. The circuit is essentially a frequency-sensitive voltage divider. At high frequencies the output behaves as if it is shorted while at low frequencies the output appears as an open circuit.
Figure 3.2: RC low-pass filter
Mathematically we have
The approximations are
At the corner
Therefore
is the corner frequency of the filter. At the corner frequency
We say that the output is down by 
at the corner
frequency.