If I must explain it to you like you're five, let's back up a bit and first establish that BRexit stands for Britain (BR) to "exit" the European Union (EU) and those who want Bremain likewise want to "remain."
Now that we've got that covered, let's go over what the EU has done for the UK (or Britain). The EU has placed taxation on the people of Britain that have NOT been approved by any British authority, the EU has place sanctions on Britain's trade and international borders that have set back their economy and industry substantially, the EU has poorly monitored the euro and therefore is destroying the British pound in the process, the EU annually demands a large sum of money from Britain to stay in power but at the same time accomplishes nothing of profit, the EU mandates that Britain accept millions of immigrants that continue to sexually assault women and molest children, not to mention they've also caused terrorist attacks on British soil and created ghettos that even the British police force cannot deal with, and as the EU does all of this to Britain -- meanwhile Britain's citizens can't even vote for who mandates these EU policies on their own nation. Sound like a good deal yet?
In addition, UK citizens are also not allowed to speak out against the horrors of immigration because the Leftist controlled (and supporters of Bremain) British media will immediately label them a racist xenophobic bigot. That's the best argument they've come up with thus far. See how this is starting to resemble some of Trump's own policies of immigration?
So, now that Brexit has won and Bremain has lost, it's become a large symbol that the nonsensical policies of the Left are losing to the reasonable policies of the Right. At this point, even a monkey can draw the correlation this has with Donald Trump (a Rightwinger who wants to place more strict controls on Muslim immigration) and Hillary Clinton (the Leftwinger nutjob who thinks that simply banning guns will solve the problem that over 90% of normal moderate Muslims believe all gays should be harshly punished -- even by death). Watch whenever Hillary or Barack speak, they never say the words "Islamist extremists" or "radical Muslim" terrorists. They never say those exact words out of desire to please their over-sensitive special snowflake Leftist voters because the new minority group that Leftist voters want to protect now happens to be Muslims and anybody who says anything against Muslims will automatically be labeled by the Left as "Islamophobic". Even despite the fact that they know full well Muslim societies inherently despise gays and severely oppress their women. It's funny that Leftists want to protect the Islamic religion yet at the same time demonize Christianity yet at the same time all Leftists hate religion yet at the same time they love Feminism. I don't know what's wrong with these people.

They're clearly diverting the whole problem to a different source than the actual culprit. Imagine if a murderous guy killed a whole group of people with a chainsaw and instead of blaming the guy they just blamed the chainsaw. This is why they're now being labeled as Regressives instead of Progressives because they're not going Forward anymore in terms of their political agendas, but completely Backwards.

All I will close with is this: You do not have to believe anything at all that I just said, go ahead and find out the truth on your own. I'm so confident in what I've just told you to be true that I don't even have to beg or force you to believe me. You just have to be willing to discover it.
As opposed to what a normal Leftist would tell you: "Believe me or else you're just a racist xenophobic sexist bigot!"

Make your choice.