the quesition you ask is a good one but very difficult to answer in general. The French bien has many different meanings in conjunction with other verbs and sometimes changes the implication of connotation of a sentence subtly.
And then there is the idiomatic use of bien, as in, e.g., être bien mal -- to be close to death. A small collection of bien and its uses can be found here: https://www.lawlessfrench.com/expressions/bien-expressions/
I don't know of any rule you could memorize which would teach you all you need to know about bien. Just read and listen a lot and you'll pick it up eventually.
-- Chris (not a native speaker).
Ce chapeau va vraiment bien à Margot. -- This hat suits Margot really well.
So here is a more literal translation of the French. As you can see and as Alan pointed out, vraiment is "really" and bien is "well". So in this example, bien is used just in its function as adverb of bon.
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