Recall that the P wave indicates atrial depolarization (initiated by firing of the SA node ... spreads to the AV node where there is a slight delay before the electrical impulse is conducted ...

The SA node generates an electrical impulse that causes the upper heart chambers (atria) to contract. The signal then passes through the atrioventricular (AV) node to the lower heart chambers ...

initiated by firing of the SA node. The atrial depolarization eventually spreads to the AV node, where there is a slight delay before the electrical impulse is conducted to the ventricles.