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What does the "Expiration Date" indicate under the "User License" section (located in Company Information)?
It is the expiration date of those licenses. You should contact Salesforce if that date is near.

Different licenses can be purchased at different intervals, so when they are due for renewal salesforce show this as under Expiration Date. If not renewed user access with those licenses is seized for time being, till renewal happens

Do you know which timezone it expires? GMT or PST or Local Time ?