Well the rallying support bit was a bit complicated.

In what way? It's been two years of Ukraine doing nothing but that.

I also didn't understand what HMMV meant at first.

It's a typo. I didn't proof read my comment since it was pretty late already. A HMMWV is a very common US military vehicle. Probably one of the most well known & iconic ones.

That whole thing about a red line confused me at first. I thought you meant the geographical front line of the conflict.

No. A red line is a figurative phrase about something that shall not be crossed - or else... Russia put up many red lines over the course of the war, including their borders being crossed, which would result in a nuclear response (which is typically the usual warning for pretty much any red line). There were many others, such as weapon deliveries to Ukraine, specific weapon deliveries to Ukraine such as tanks, or missiles, where those weapons could be used (specifically "if you use them to strike within Russia then...") etc.

Also it's still not 100% clear to me what reaching a critical mass means in this context.

Critical mass: An amount or level needed for a specific result or new action to occur. In this context it would mean that Ukraine gets enough gear to be able to achieve their military goals of fighting back Russia and push them back behind their borders.

Generally speaking the sentences were a bit advanced and seemed to hedge on someone understanding military stuff and having a pretty extensive background on the conflict. I had to reread it attentively to understand, which is not what I'd generally expect of an ELI5 reply.

Well, if you ask about a conflict that goes on for two and a half years, then I'd expect you to at least know the basic premise of it. You can't expect an ELI5 to cover that much time with all its events so of course I focused on the main part of their question. I would not describe my secondary English skills as "advanced" though. I speak very simplified English and don't use any complicated words.

There's also that whole thing about foreign weapons and their suppliers having some degree of control over what Ukrainians do with them, which wasn't obvious to me.

Specific export rules are standard for weapon deliveries and not really exclusive to weapons delivered to Ukraine. That topic has been talked about pretty much every month for the past couple years.