The 250 ms before the stimulus is used as a measure of “baseline” activity. For each epoch (time window before & after the stimulus), the baseline is subtracted from the waveform. This leaves only the activity that occurred in reaction to the stimulus, and removes the other background neural activity that was going on at the time.
When you epoch the data, it automatically asks you if you want to remove the baseline as well. You can do it then, or do it later using these instructions.
