A visa is a document or actually a sticker inside your passport that allows you to enter the US from a foreign country, much like a ticket to a movie theater. You must apply and obtain that visa (ticket) at the US consulate/embassy (box office) in your home country before coming to the US (entering the theater).

A visa is only used to enter the US, but doesn’t say anything about how long you can stay. To stay in the US, you must have legal status.