One is articulate if one speaks articulately. Most definitions say ‘speaks clearly’. However, the term “clearly” itself is subjectively inarticulate. To speak articulately is to express concepts as a unified set of causal relations. The problem with any unified set of causal relations is that the set of causal relations a) must be expressed as a series of actions (to avoid the platonic or subjective errors), b) must bridge the gap between different audiences with greater and lesser knowledge. c) and the steps in those causal relations must be short enough that the audience can follow them.
Net is to articulate something well requiers we understand it well. and understand others well enough to calculate how we can bridge the gap between those states of understanding.