A more thorough explanation:

Definition: To bind by legal or moral duty; obligate. To bind by doing a favor or service.

Example 1: The law obliges citizens to pay taxes.

Example 2: My friend asked me to help him move, and I obliged.

Example 3: The company obliged its customers by offering a discount.

These examples illustrate how oblige can refer to both legal or moral duty, as well as voluntary actions done as a favor or service. In example 1, citizens are legally obligated to pay taxes. In example 2, the speaker voluntarily helped their friend move. In example 3, the company voluntarily offered a discount as a service to its customers.