It's helpful to understand the derivation of the measurements behind those scary little boxes. This will make it easier to answer questions (your own and anyone else's) about what they mean.
The big boxes on an EKG tracing run under the pen at 300/boxes per minute. That means if there is one (regular) QRS in every box, the rate is...300/minute (ouch). If in every other box, then, that would be...300/2 = 150/minute. Every third box, 300/3 - 100/minute. One every 5 and a half boxes? 300/5.5 = 54.5 (we call this "55"). And so forth. If there is one every tenth box, the rate is 300/10 - 30/minute (ouch again).
Now, to measure intervals in the tracing that tell you how long it takes the electrical impulse in the heart to get from point A to point B (or, say, the SA node to the AV node, which is the PR interval, the impulse going through the atria), you count the little boxes. A little simple division (one big box = 1/300th of 60 seconds, so....) will get you the useful information that every big box is .20 seconds and so every little box (there are five in every big box) is 0.04 seconds.
So then if the PR starts at the beginning of one box and the QRS starts five little boxes later, that's 5 x .04 = .20 seconds (whew, normal). If the PR is four little boxes long, it's .16 seconds. If it's 8 little boxes long, the PR is .32 seconds (waaaaaay too long, 1st degree heart block by definition).
The distance between the beginning of the QRS and the end of the QRS tells you how long the impulse takes to get through the ventricles. It's usually around .08, about two little boxes wide. If you have a big honking wide weird looking QRS that takes a long time, that tells you that the conduction is not going through a nice normal pathway down through the right and left bundle branches the way it should, and is instead taking detours around infarcted areas OR the electrical impulse started somewhere entirely different from the AV node where it ought to have come from....a PVC, premature ventricular contraction.
Delayed conduction can be caused by any number of things (this is where your memory about drugs that affect conduction comes in).
Also, being able to measure PR intervals across a strip gives you useful information. What if they are different? Are they all randomly different? Are the Ps regular at rate X and the QRSs regular at rate Y, but there is no relationship between them? 3rd degree heart block (AV node is toast). Is there a normal PR on the first beat, a little longer on the next one, a little longer on the next one, and then there's a P right on time but no following QRS? This is one kind of 2nd degree block, where the AV node runs outta gas after a few conduction jobs and has to regroup before going back to work on the next cycle.
If there are no decent Ps to look at, just sort of a staticky baseline, and the QRSs are irregularly irregular, that's atrial fib. You know this because an organized atrial contraction would draw a nice P wave, and there aren't any, and the AV node isn't getting regular messages from above, so it does its thing irregularly.