A tact simply is a label for something.

A tact (a word derived from “contact” with the environment) occurs when you see, smell, taste, hear, or touch something in your environment, and then label it. When you tact something, you're not responding to someone asking "What is that?" you're responding to the thing itself.

Tacts are reinforced by social interactions, such as praise or attention, rather than access to the thing itself.

Example of a tact:

A child sees a car pull into the driveway.  

Child:  ”Car.”  (Tact)

Mother:  ”That’s right, that’s Grandma’s car!”  (Reinforcement)

The car pulling into the driveway is the "something" that the child is responding to (this is called a non-verbal antecedent in behavior analyst-speak), and the child's tact was reinforced by attention and praise from her mother.

If you want your child to speak more, and in more complex sentences, teach them as many tacts as possible. Tact things in your environment, sounds, smells, tastes, and actions. The more tacts your child has in that brain of theirs, the better their communication skills will be.