When you come across a word you don't know, don’t stop reading. You can often figure out the meaning of an unfamiliar word by using the words around it. The surrounding sentence or paragraph is known as the context.
Here's an example: Sue's boss was a callous man. He did not react to her tears and apologies.
From the context you know that a callous man does not react to tears and apologies. Callous means “hard-hearted” or "insensitive to emotion."