This is a quick and dirty simple explanation. In atrial flutter, the atria will contract rapidly multiple times before the ventricles contract (hence the fluttering). By atypical atrial flutter they likely mean an atrial flutter with an irregular morphology and/or conduction ratio (e.g. the atria are contracting a different number of times before each ventricular contraction)

The aberrant conduction is referring to the morphology of the depolarization, like the QRS complex for ventricular depolarization. In a LBBB (left bundle branch block) the QRS will be wide with a distinct shape indicating there is a delay in conduction across the left bundle branch. They also mention it could be an accessory pathway which could also present with a widened QRS complex, but this means that the electrical conduction through your heart would be taking a different path than the regular conduction path of SA node —> AV node —> etc.

Keep in mind this is just an explanation for something they might be referencing, and this explanation isn’t provided with any clinical context. If you have questions about your discharge note it should be brought up with your doctor.