Every plane has a "transponder" which is an electronic device for very simple, two-way, basic data communication between the plane and radar systems. Radar/ATC systems "interrogate" the plane (via a signal that essentially says, "give me your info"), the plane responds by transmitting some basic data -- a specific 4-digit numeric code (which the pilot sets), plus some basic data about the plane (it's position, it's altitude, it's ID, etc.). The data exchange typically takes place every few seconds (depends on location -- some regions have minimal coverage). All of this takes place in the background, it's automatic, once the transponder is turned on and set up.

Most of the time, aircraft are assigned a specific "squawk" code by ATC (the term "squawk" comes from the early days of the technology; when transponders were first used, pilots referred to them as "the parrot"); pilot sets the squawk code and lets it do it's thing. The code is changed from time to time based on instructions from ATC. There are a few other "generic" codes for specific situations -- for example, 1200 is the code for aircraft not using the ATC "system", by choice (for example, a small Cessna out flying around for fun on a nice day, where there isn't a lot of other air traffic, far from any big busy airports). 7600 is the code to enter if you are (or were) talking to ATC but you've lost communications.

7700 is the generic transponder code for "emergency". I suppose this might be a quiet, discreet form of protest by Russian pilots, making a statement about what's happening to their country.

While many Russians believe the government propaganda about the war in Ukraine (typically the older generation, and folks who only get their news from state TV), many are dismayed by recent events and some dare to protest against the war and the direction of the country (often, but not always, it's the younger generation, and those who have access to international news sources and unblocked internet). I would expect some Russian pilots fall into both categories -- just like anywhere, opinions are divided. Unlike most places, in Russia you immediately get arrested if you hint at holding a view that questions the government policy. Protesting is pretty dangerous there.

I'd like to think that Russian pilots squawking 7700 is their way of telling the world that they don't agree with what their government is doing. Like flying a flag upside down.

OTOH, it could be that they just don't have any assigned code or a more appropriate one to use, and they do have to pick some number. But alarm bells should go off if any plane is squawking 7700 (at least where I live). So if this is a political "statement" then people working the Russian ATC system must be in on the "joke" too or have some understanding to ignore it (which is not a safe practice....see "the boy who cried wolf" et all...)

I guess I prefer to be more hopeful, and I choose to see this as a sign that many Russian pilots oppose the direction their country has chosen, and this is their small way of broadcasting that opinion, in case anyone (other than their government) is watching.

ETA: One Russian pilot's not-exactly subtle opinion: "The war in Ukraine is a crime, we must stop it immediately"...
From DailyMail.com: Russian commercial pilot announces after flight lands that 'the war in Ukraine is a crime, we must stop it immediately' - to rapturous applause from passengers
Cajones, brother. Squawk 7700, wishing you blue skies and tailwinds.