The I-94 is your arrival record. US Customs and Border Protection (CBP), the officers at the airport, create it when you enter. Today it is electronic, so nobody hands you a paper.
It shows three important things:
Class of admission: your status, for example F1, H1B, L2, B2 or WT (ESTA).
Admit Until Date: the last day you may stay. For students and exchange visitors it often says D/S (duration of status) instead of a date.
Most recent date of entry.
Your employer, school, the Social Security office, the DMV (the office for driver's licenses) and USCIS will ask for it.
Open i94.cbp.dhs.gov on your phone or computer. You can also use the CBP Link app from CBP.
Choose Get Most Recent I-94.
Type your first name, last name, date of birth and passport number exactly as they are in your passport.
Choose the country that issued your passport.
Save it as a PDF and take a screenshot. Email a copy to yourself.
If it says "No record found", try again with small changes: first and middle names together in the first-name box, or your old passport number if you entered with a visa in an old passport. Records can take a day or two to appear.
Compare the I-94 with your passport and visa:
Name spelled the same as the passport.
Class of admission matches your visa (an H-4 spouse should not show H1B; an L-2 spouse should show L2S; an E spouse should show E1S, E2S or E3S).
Admit Until Date makes sense. An F-1 or J-1 student should see D/S. A worker usually sees the end date of the petition, or sometimes an earlier date, for example when the passport expires soon.
If something is wrong, fix it now. See Fixing a mistake on your I-94.
Thinking the visa in your passport decides how long you can stay. The I-94 decides. See Visa stamp vs permission to stay.
Using the OMB date at the top corner of the PDF. That is a form date, not your date.
Waiting months to look. A small error can block a driver's license, a job start or an extension.
Download a new copy every time you come back from a trip abroad. Each entry creates a new I-94.