Articulation refers to the actions of the lips, tongue, jaw, and palate in producing the sounds of speech.
Some children experience difficulty acquiring these actions correctly, and the result is speech that is unclear (and sometimes even unintelligible). This is known as an articulation delay or disorder, which comprises 50-80% of all communication disorders!
Keep in mind that a child's speech should be easily understandable by the time they reach age 3. While the presence of some errors in articulation is normal, a child should be able to make all speech sounds correctly by age 7.
Typical progress looks like...