This is an irregular narrow complex rhythm with group-beating. Sinus P waves can be seen (R3, R7 and R11). The rest do not have P waves and mostly had inverted P waves just after the QRS (inferior leads). Those complexes with no upright P waves show shortening of the RR interval.
What does it mean?
The rhythm is sinus but the junction fired earlier and a faster rate and gained control (usurp) of ventricle. The shortening of the RR interval would mean that there is a progressive delay of the junctional impulse as it exits the AV junction or there is type I (Wenckebach) exit block.
Image 2 ladder diagram
With the featured strip, we could say that the sinus rate is less than 88 bpm because the RR interval between the sinus beat to the junctional beat is about 0.68 sec (~88 bpm). In this case, the junction was able to gain control the atria as manifested with an inverted P waves. The reason why no inverted P wave can been on some junctional beat is that the ventricular depolarization also happened during atrial depolarization.
Thus, this is SR + accelerated junctional rhythm (or most likely junctional tachycardia) with type I exit block.