I have a noisy ECG Signal. want to use PSD to analyse the Signal to noise ratio
Why it is plotting against only 500hz and what information I can get on this??



Plotting against $500Hz$: Because that is the frequency of the sampled signal. We usually sample a signal at least two times of its max frequency in order to avoid aliasing. In your case $fs=1000Hz$ (sampling frequency), so max frequency is (no higher than) $500Hz$.
The information you can get: Which frequency corresponds to high powers. You can avoid power below $0$ frequencies. High frequency components are usually our signal while low power is noise.