By talking to them, such as explaining actions as you do them, naming things in places you are at, properly introducing them to people, etc. you help them understand the purpose of language.
By reading to them you expand their knowledge of language even if what you are reading is far above their age level. It does wonders for their ability to begin communicating verbally.
These things are really crucial, but if a toddler's speech seems delayed, most often all you need to do is be patient as you continue the above. A pediatrician could help you know whether you need to explore something out of the ordinary.