Can someone explain Apraxia to me?

My son is almost 4 and has to take speech therapy. Last year at this time he was tested and had the speech of a 12 month old. A month ago he was retested and he now has the speech of a 15 month old child. His speech teacher told me she was concerned that he could have Apraxia. She advised me to take him to his doctor and get him to a Neurologist. I just want to know what i am looking at. Also if anyone with a child that has this problem could answer this question i would appreciate it. I am worried.