Simply put, it is meaningless. It's a term that emerged the in the 18th century to describe the system of fief-holding in Ancien Régime France. It was then applied to the way that fiefs were held at other points during the middle ages. The problem is that it condenses the political systems of more than a thousand years of European history into a single term. The political society of 8th century Britain (the Anglo-Saxon kingdoms before the Viking invasions), 13th century Germany and 15th century France were extremely different from on another.
During the middle ages, from the Carolingian dynasty onwards, an idea of vassalage and fief-holding developed, as opposed to the allodial tenure of land -- before the 9th century or so, land was not held from a lord but in one's own right. This slowly changed during the Carolingian Empire and its breakup, so that land often came to be held as a vassal for another lord. Roughly around the same time, the manorial system developed so that peasants were enserfed. The decline of the European monarchies after the fall of the Carolingians (the deposition of Charles the Fat in 887 being a/the key event in that process) lead to counts, dukes, marcher lords and bishops taking more authority for themselves rather than theoretically being granted this authority from the king. What developed in Western Europe in the proceeding centuries became know as feudalism, as greater lords imposed their authority in their respective regions, taking pledges of loyalty from lesser castellans. However, this process was not at all uniform, as many parts of Southern France, Italy and Iberia never developed these concepts of vassalage.
However, medieval political society was in fact not static, but dynamic. Things were always changing, looking completely different in different centuries. For instance, Late Medieval France and England might have been said to be "feudal" but both had relatively strong states by then, able to raise taxes and fund armies (France's state development was helped by pretty much collapsing twice during the Hundred Years War and having to be rebuilt in order to survive).
So, as you can see, "feudalism" is a nebulous term that loses any meaning when actually studying the intricacies of medieval political society, and often obscures the truth of the matter, as one might say -- how could Carolingian king view a count as an office holder and public official when he was really a feudal lord? How could Parliament be granting Edward III taxes for his war in France when he should really have been leading a feudal host instead of an expeditionary army that was paid wages in currency? It is instead best to talk about the facts of a specific period, and understand the complex and nonbinary aspect of the fief-holding system at different points during the millennia or so we call the middle ages -- which of course our ancestors during the period were not even vaguely aware they were living in.
Hope this helps.
Sources (just to name a few):
France in the Middle Ages by Georges Duby
The Carolingian World by Marios Costambeys, Matthew Innes, and Simon Maclean
The Hundred Years War vol. I-IV by Jonathan Sumption