Avast Free Antivirus is, as its name suggests, a free antivirus.
However, you may have noticed that Avast informs you (after a year) that your Avast Free Antivirus license is about to expire.

Don't panic, Avast Free Antivirus remains free for life.
However, it simply requires you to renew your free version of Avast every year. This certainly allows them to know that you are continuing to use their free antivirus on your computer.

To do this, at the bottom of the Avast Free Antivirus interface, simply click the "Renew" button.

Important : always choose the free option and don't be fooled by offers for paid versions of Avast, where the paid button is always more prominent (green here) than the free button (gray here).
In this case, click the gray "Select" button at the bottom of the "Avast FREE Antivirus" column.

As you can see, Avast thanks you for choosing the free version.

Warning : it's trying again to upgrade you to the paid version by displaying a large green "Start Free Trial" button (which means switching to Avast Premium Security, which is PAID).
You can either close the window or click the small gray "No thanks" link (which allows you to stay on the free version, hence why Avast doesn't make it very prominent compared to the large button above).

In short, you'll see that you still have the free version of Avast Antivirus and that the expiration message at the bottom has disappeared.

If you wish, you can go to the menu in the upper right corner (to better understand why you need to renew your free version of Avast).

Go to "My Subscriptions".

As you can see, Avast considers you to have a subscription to "Avast Free Antivirus".
Which is obviously free; you don't pay anything, but this explains why you have to renew the free license every year.