How can I read the graph of a low pass filter
This is a common plot type to show the frequency response of a system with a single input and a single output.  Audio amplifiers, for example, are often characterized in this way.

Since the purpose of the graph is to show what the system does as a function of frequency, it naturally has frequency on the X axis.  However, what is not obvious unless you are used to looking at these graphs, is that the frequency is plotted logarithmically on the X axis.  Each tick on the X axis represents a frequency multiple, not a frequency increment.  It is irresponsible to not clearly say so, but unfortunately that happens too often.  For example, each tick on the X axis may be one octave (multiple of 2) or one decade (multiple of 10), or some other number.  At least in this case the slope of the dotted line is explicitly labeled as -6 dB/octave.

The Y axis is the gain of the system.  However, just like with the X axis in this example, the gain is also shown logarithmically.  This time though, that is clearly indicated with the legend "dB", meaning "deci-Bels".  dB is used to express power ratios.  Specifically dB = 10*Log10(Power2/Power1).  Since power is proportional to the square of the voltage, we often use dB to express voltages ratios: dB = 20*Log10(Voltage2/Voltage1).

Note that 0 dB always specifies a ratio of 1:1, which means the output voltage just follows the input voltage.  In this case the system is a single-pole R-C filter.  At low frequencies it leaves the signal alone, and at high frequencies it attenuates the signal ever more with frequency.  That's what R-C low pass filters do, and also what the graph is showing you.