Can anybody explain me how pop_interp function works.

It takes three parameters , eeg data to be interpolated, original eeg file (I can't understand which file is this) and method. On which basis it does interpolation. And eeg data is the dataset or the channel file. Similarly original eeg file is a dateset file or channel file? From where we get original eeg file.

[Matt J]

What toolbox is it in?

[Syeda Noor]

EEGLAB toolbox in matlab. Command is:

EEG = pop_interp(EEG, channels, method);

[Ive J]

This is not a native MATLAB toolbox, you'll probably find more here:

https://eeglab.org/tutorials/