Let's say that Katelyn has gone on maternity leave and will be gone for at least several weeks. We can lock her account with:

sudo usermod -L katelyn

When you look at Katelyn's entry in the /etc/shadow file, you'll now see an exclamation point in front of her password hash, as follows:

katelyn:!$6$uA5ecH1A$MZ6q5U.cyY2SRSJezV000AudP.ckXXndBNsXUdMI1vPO8aFmlLXcbGV25K5HSSaCv4RlDilwzlXq/hKvXRkpB/:17446:0:99999:7:::

This exclamation point prevents the system from being able to read her password, which effectively locks her out of the system.

To unlock her account, just follow this:

sudo usermod -U katelyn

You'll see that the exclamation point has been removed so that she can now log in to her account.