Here are some examples:
* "I was going to go to the movies, but I went to the park instead." (The park is the substitute for the movies).
* "Instead of eating pizza, we had tacos." (Tacos are the alternative to pizza).
* "She didn't answer the phone, so I left a message instead." (Leaving a message is the alternative to speaking directly).
In essence, "instead" tells you that one thing is being done or chosen over another.