A tube power amplifier (like the final stage in your Mark V) has to have an output transformer to lower the output impedance (the "Ohms") to a level appropriate to drive a speaker. What does that mean? Well if you know what voltage and current and power are, then one way to look at impedance is how much of the power you are putting out is in voltage and how much is in current? That ratio is one interpretation of impedance.
The ratio of voltage to current coming out of power tubes is relatively high. But you need lots of current to make a speaker move, so you have to trade some of that voltage in for more current. That's what a transformer does. How much do you trade in?
The answer to that question is chosen by the amp designer and is your amp's rated output impedance. That means your amp is designed to put out its rated power with a specific ratio of voltage to current. The amp will pretty much put out how much voltage it's putting out in any situation, and it's designed to put out a certain amount of current along with that. Designed meaning that if it is not allowed to put out all the current it can, then you won't get the full rated power, meaning your sound will be quieter and maybe less clear. However, if your amp is allowed to put out more current than it is supposed to, then bad things can happen.
Current creates heat, more current is more heat, and too much heat can melt things. Like your output transformer. If your transformer isn't the first to go, then there are plenty of components in the amp that can be melted or just destroyed in other ways by passing too much current.
So you have to hold your amp back from running crazy with current. This is what a speaker's nominal load impedance number is meant to indicate. It's not a precise number, but it's good enough for making sure you're not letting your amp run away with itself and overheat.
Here's the short short version: If your speaker cabinet's nominal load impedance is equal to or greater than your amp's rated output impedance, you will be safe. Safe at least in terms of not overheating your amp. If the speaker's impedance is higher, as stated before you won't get the rated output power and there may be some tonal changes. There are lots of interesting interactions between a speaker cabinet and an amplifier, so how the sound will change isn't so easy to predict, but if the speaker impedance is higher, you can safely experiment and found out how it sounds.
One more thing to check is that the rated power dissipation of the speaker cabinet is greater than the rated output power of the amp. That way all the current coming out of the amp won't melt the speaker.
If you ever get into live sound PAs, just know that it's a different world because those amps are not tube amps. You still need to pay attention to power levels for sure, impedance not so much, and the rules of thumb will be different. But that's not relevant to your question. I just didn't want you to think back to this years later and think you can set up your PA the same way as your Mark V.