A more thorough explanation:

Definition: The source is the person, thing, or place that is responsible for creating or causing something to happen.

Examples:

• She was the source of the information.
• The side business was the source of income.

These examples show that the source is the originator or primary agent of an act, circumstance, or result. In the first example, "she" is the source of the information, meaning that she is the one who provided or created the information. In the second example, the "side business" is the source of income, meaning that it is the primary cause of the income.