Ukraine was once part of the USSR although before that it was an independent state after the Russian Revolution.
Putin has based much of his image on Restoring Russian pride after the fall of the USSR (much like Hitler did with Germany on his rise to power).
So he annexed the Crimea a few years ago, which was an obvious precursor to this invasion. He also took Georgia in 2008 (although I don't know much about that).
Ukraine also holds strategic importance for Russia.
Additionally, Putin feels his regime is threatened by the gradual expansion of Nato as more Eastern European countries (and countries also now free of the USSR and valuing democracy) join.
There is some argument that a gentlemans agreement was made between the west and Russia that Nato would never expand to a point where it would threaten Russia but this isn't much of a case for invasion.
Democratic countries fear the Ukraine invasion is a signal of empire expansion in the same way Hitler sought to expand the Third Reich beyond Germany and conquer Europe last century. Given that China is believed to have similar aims, there is also a widespread concern that China is watching to see how the West reacts to invasion of Ukraine. China has made no secret of the fact it denies Taiwan's sovereignty, which is under constant threat of invasion from them.
While there is some argument that Russia lacks the resources to launch a full scale war against Europe, they do have nuclear weapons and many people believe Putin is unhinged enough to use them. There is also the chance of an alliance between China and Russia should the situation escalate.
While democratic Western countries have pursued peace, defended their military and taken Russian & Chinese money (in exchange for power and influence in their counties), China has been increasing it's military might on a huge scale in recent years. They've also been creating man made islands in the Pacific which give them strategic strongholds and are laying claim to large parts of the waters in those areas.
In the meantime, the West has been ravaged by financial crises and the pandemic and Western alliances have been challenged by Brexit.
People are terrified that if the situation escalates, Nato could be drawn into a war of a huge scale and we feel woefully unprepared.
There is much more to it, in terms of history and nuance and debates over what has had the greater affect on leading to this situation. Also debates about how likely the worst case scenarios are to occur.
I'm not sure why the same fears didn't play out with the invasion of Georgia, but this current aggression from Putin is certainly seen as a threat to everyone.