In case you don't know your onions ... Royal Ascot is a historic horse racing event in England that's been going on since the 1700s ... with everybody from the Royal Family to huge celebs in attendance.

Seriously, the Queen and Princess Beatrice are both there -- so you know it's a big deal.

Of course, there's a strict dress code that requires everyone to style out their noggins with massive head wear ... which explains why the Seahawks superstar looks like the Monopoly guy.

There's big money involved on the track -- with roughly $10 MILLION in prize money. No word if the Queen ever places any bets -- but it'd be weird considering she'd be using money with her face on it.

But, the biggest shocker of the day ... one of the dudes in the royal procession FELL OFF HIS HORSE -- but, don't worry, everyone makes mistakes ... right, Russell? (#HandItToMarshawn).