Building background knowledge can seem daunting, but try this incredibly easy activity from Reading Rockets.
Give your child a word and have them try to give you words in the same category. For instance, show them a banana at the grocery store and ask them to name other fruits. Try it with toys while you're playing, vehicles as you're driving, tools at the hardware store or dishes at a restaurant. Pretty soon your child will have a wealth of new vocabulary and know more about their environment!