The ECG tracing shown below tells you three important things:
1) This patient has FBI tachycardia (Fast, Broad, Irregular). This means, the patient has atrial fibrillation with integrate conduction via an accessory pathway (AP). Therefore, you know there is an AP present in this patient.
2) Since the ECG shows right bundle branch block (RBBB) morphology with right axis deviation (RAD) the AP must be located on the left side, most probably left lateral. Left sided preexcitation of the ventricles causes RBBB and RAD because the left bundle is activated later. The opposite would be the case in a right sided AP (LBBB & LAD).
3) In preexcited AF (i.e. FBI), the shortest preexcited R-R interval (SPERRI) is used to define a pathological AP (i.e. SPERRI <250ms). In this ECG the shortest R-R interval measures 200ms. Therefore we now know this is a pathological high risk AP that needs to be ablated. And we already know where to look for.