Children gradually learn the meaning of words through experiences that occur when the word is spoken. Vocabulary knowledge is really a complex set of associations between the sound of a word and memories, related concepts, and even similar sounding words. A new vocabulary word is not simply acquired one day with enough repetition; vocabulary associations are built gradually and the meaning of every word grows and changes with word experiences.
We learn words within social experiences. A baby hears the word Daddy in the presence of one big hairy guy that grins and cuddles and threatens to eat her toes and the word Daddy gets connected in the babies brain to the feel of a beard, the sound of a deep male voice, the silly grin and the sense of being loved. Every experience that this child has with her father becomes part of the meaning of Daddy. Gradually the word Daddy is understood as more than a label for one beloved man. She knows that Daddy can refer to other people's father. The word can function in many different ways. It can bring her father to her. Daddy is the word this child utters to call her dad when he is in a different room. It can function as a protest or complaint when her father sends her to bed and she does not want to go: Daaadeee! Sixty years later, as I know personally, the word Daddy will serve to bring her father to mind when he has been gone from this earth for a decade.
We learn words within social experiences. I was in The Netherlands in August for long enough that I began to understand some Dutch words. The first time a store clerk asked me in Dutch if I needed a shopping bag, I did not understand and she switched to English and repeated herself. By the time I left, three weeks later, I did understand and would shake my head no and pull out the bag I carried with me. I learned the words in that particular place and situation but now, here in the United States, I have no idea what the word for shopping bag is in Dutch. If someone said the word to me here, I don't think I would understand it.
Children with autism may have difficulty learning vocabulary but like all learners, they need to learn vocabulary in a variety of situations in order to learn the word. In fact, it appears they learn new vocabulary like I learned the word for shopping bag in the Netherlands. They tend to learn the word in one situation but not understand the word in a different situation. When you want to help your child learn the meaning of a word or phase, use it in many different situations over many days and even with different people. Use it in different places within a sentence. Use it in sentences that are meaningful to your child.
Example of different contexts on might use to teach the meaning of 'shopping bag':
Let's go buy a new toy and put it in this shopping bag.
Help mommy take the cans out of my shopping bag.
I can't find my shopping bag--help me look for my lost shopping bag.
The shopping bag is too full, we can't carry it.
You carry this shopping bag, my hands are full.
I have a surprise for you in the shopping bag!
Now if someone in The Netherlands had used the Dutch word for shopping bag in all of these contexts, I would probably still know how to say it today.
Ok, now I am curious. I will look it up.
"Boodschappentas" is the Dutch word for a shopping bag that is used for groceries. No wonder I forgot. That is a long hard word.