What's the noise in this signal ? (Beginner question)
From the spectrogram (frequency domain plot) you have a large signal at 60 Hz and harmonics. This will be mains pickup if this was recorded in a area with a 60 Hz mains supply. On the upper time plot this signal has a peak to peak amplitude of about 0.5 units. (Note that there are 30 cycles over 500 ms.)

It looks like to me the isolated peaks are the noise.

As JRE and MBaz suggest, it helps to know what your wanted signal looks like. The Wikipedia article on ECG gives an idealized view of a ECG trace (as well as some actual plots). You should be able to see that the isolated peaks are actually part of your wanted signal. Your patient seems to have a pulse rate of about 75 bpm.
All your wanted signal seems to be below 25 Hz. To get rid of the mains interference one way would be to low pass filter the signal or use a comb notch filter to remove 60 Hz and its harmonics. A notch filter may have less effect on your wanted signal but whatever filter is used you need to consider how its amplitude and phase response will affect your wanted signal.
An alternative approach is subtraction. You generate a local copy of the interfering signal and subtract it from the raw input leaving behind the wanted ECG. As this isn't based on a filter there are no unwanted amplitude or phase effects on the signal.