Heart words are words that have an irregular spelling to them - in other words they are hard for kids to sound out since part of the word does not follow the rules we know about letters and sounds.
So the part of the word that is irregular needs to be memorized or learned "by heart."
An example of a heart word would be "what." The "a" in that word does not make its normal short or long a sound. So your child needs to know this part by heart. So when we learn this word, we would put a small heart over the "a" to remember that the sound is not what we expect.
Click here to see some more examples of heart words. These videos are excellent for practicing these words.