I read in the internet that Accelerometer suffers from 1/f noise at low frequencies and Gaussian noise at high frequencies. But i didn't understand what frequency they are referring to? Is it the sampling frequency of the accelerometer? or operating frequency of the mobile phone processors(I was looking for MEMS Accelerometer in android phones)?or frequency of some other signal(if yes then what kind of signal)?
They're talking about the frequency of the input signal. Since this is an accelerometer the input signal is movement.
Fast, high frequency vibrations suffer from Gaussian noise. Slow, low frequency motion suffer from drift. This limits the range of motion you can accurately measure with accelerometers.
What does frequency refer to when it comes to motion? It simply refers to the change in direction of motion, or more specifically the change in direction of the vector of motion.
It may be difficult to imagine that linear forward motion has a frequency but linear forward motion only has a frequency of zero if the object either travels at constant speed or constantly accelerates for eternity. For a car or a train or a plane or a boat or a spaceship this is not the case. All vehicles in motion has to stop eventually. When it stops, it has completed a motion with frequency of 1/(2*time_of_journey).
For accelerometers, the frequency is more since accelerometers measure acceleration, not distance. So if the vehicle travels like a spaceship which accelerates only once and decelerates only once then the frequency of the input will be 1/time_of_journey (since the value of acceleration starts at and returns to 0). But for cars and boats and bicycles and most other vehicles the frequency is significantly higher since most vehicles constantly accelerates and decelerates. So the frequency of acceleration is generally 1/(time_of_journey/number_of_times_you_hit_the_brakes).
Seriously, the numbers for this is generally very low. Take the simple example of a spaceship going to the moon and ignore the launch and landing. The frequency of acceleration for Neil Armstrong's trip to the moon was 1/4 days or 0.000003 Hz. That doesn't look like much of a frequency.
So why state the parameter in terms of frequency? Well, in signal processing motion is generally assumed to be sine waves. It's not realistic and doesn't reflect the real world but is good enough and simplify things enough to the point where we can write down equations to model things. Which is why you see things like "accelerometer noise" being quoted in terms of frequency.
Vehicles also do experience acceleration at higher frequencies though. Vibrations caused by the engine and bumps in the road and friction with the air has frequencies in ranges that we recognize as being periodic. Indeed, often our ears detect such things as rattling or humming or swooshing sounds. Sensitive accelerometers detect them too.