Each class has a specific dice they throw to determine how much health is added each level up. Thus, a wizard gets less than a fighter or barbarian. There is also a stat called constitution that impacts HP in a positive or negative way.

So Caesar, as a 10th level fighter, would get a bunch of health from levelling up over and over from the experience gained fighting.

AC is armor class. It determines how hard someone is to hit / wound. Someone with plate armor has a higher armor class, than someone in a toga. Here, Caesar is frail and old, he lost dexterity, which is a stat that helps armor class. Making him easier to hit. 9 AC is really low, basically anyone can get a hit in. Imagine if he was wearing full plate though, those senators wouldn't be able to do anything to him.

They each need to roll a 20 sided dice, which with bonuses, needs to be 9 or higher for them. The bonuses come from someones strength, dexterity, whether they know how to use a dagger, etc. A guy who stabs people all the time will have an easier time hitting Caesar, than a senator who was peer pressured into stabbing him.

1d4 means "one dice throw of a four sided dice". Which is a "triangular" dice with numbers 1, 2, 3, 4 on it. It determines how many hit points are taken away if they hit and wound Caesar (along with any bonuses to damage they might have for being stronger, more dextreus etc.)

Each round, everyone gets to make a move, and a round represents around 6 seconds irl. So here, they calculated out the fact that Caesar has a lot of HP, so the geezers in the senate need to do a lot of damage in this first round, to instantly kill Caesar.

To put this damage into perspective, getting hit by lightning is 2d10 (two throws of a 10 sided die, maximum 10 each, maximum 20 overall). So the senate stabbed him so much, that it counts as a little over 3 lightning strikes hitting you at the same time, in the most devastating way possible. For more context, a random commoner is around 8 HP.