Articulation refers to the way in which we say or pronounce sounds and words. This directly relates to how easily understood we are by others. In technical terms, it is the process that sounds, syllables, and words are formed when our articulators (tongue, teeth, jaw, lips, and palate) alter the air stream that comes from our vocal cords. It is a very complex collection of actions you must complete very precisely to form sounds correctly.
By the time a child enters Pre-Kindergarten, most listeners should be able to understand at least 90% of what he/she says.
By the time a child enters Kindergarten, most listeners should be able to understand at least 90%-95% of what he/she says.
When a child cannot produce a sound that is age-appropriate, it draws attention away from the speaker's message. They are motor errors which can occur among people of any age; but it is more common in children who are still developing and/or their articulators haven't developed properly.