Automatic Visa Revalidation (AVR)

"Under the automatic revalidation provision of immigration law, certain temporary visitors holding expired nonimmigrant visas who seek to return to the U.S. may be admitted at a U.S. port-of-entry by CBP, if they meet certain requirements." U.S. Department of State

Eligible F and J nonimmigrants may depart the United States for travel to Canada, Mexico, or an adjacent island for 30 days or less. For a list of adjacent islands, visit ICE.gov.
Eligible non-F or non-J status holders are allowed to use AVR for travel to Canada and Mexico ONLY.
Individuals who have changed status in the United States and hold a U.S. visa for the previous status may still use AVR, but they are advised to carry current supporting immigration documents when traveling. For example, if you changed status from F-1 to H-1B and only have an expired F-1 visa in the passport, carry your I-797 approval notice when traveling under AVR.
Citizens of countries designated as State sponsors of terrorism are not eligible to participate in AVR.

The expired U.S. visa is considered to be automatically extended to the date of reentry provided that the visa has not been canceled, and provided that you have not applied for a new visa. To take advantage of AVR your travel must be to contiguous/adjacent entities for less than 30 days.

Additional resources:

Please note: You may require a visa to enter Canada or Mexico. For information on travel to Canada and Mexico, please visit their respective consulate websites.