Yes, anyone can "run" a "witness" "node" if that's what you are asking. Witnessing can be interpreted as an action that validates blocks on the chain, maintaining the network secure and functional. A node has others functions besides witnessing. Witnessing need to be voted (by the community) in order to have "impact" on the block validation.
Very broad question, but answering it as simple as I can, "someone that works to protect the chain" is how I would put it. And how that's achieved, it can be via many methods, being the main one, someone that is on the top20 and acts both to protect, validate it maintains its intended functionality and also contributes to develop the chain (no blockchain should be static in its development, otherwise is a dead on arrival thing).
Answering here as on X I have less "space" to answer...
So, this is an answer to your X question...
Very good questions!