A phone that allows two SIMs.
Since a SIM links a phone to a specific carrier (service provider), billing account, and phone number, a dual-SIM phone can be linked to two different phone numbers associated with two different billing accounts. Those accounts and phone numbers can be with different carriers, and even in different countries.
One common use is to have one phone with both a business phone number (and account) and a separate personal line. Another use is to have accounts with carriers in two different countries, in order to avoid costly roaming rates.
Some dual-SIM phones only allow one SIM to be active at a time, requiring the user to switch SIMs via a menu option. Other phones allow both SIMs to be active at once, a feature sometimes called "dual-active". Similarly, some that allow both SIMs to be active only allow one SIM to use certain network features like 5G data.
One or both SIMs in a dual-SIM phone may be an eSIM. Older dual-SIM phones had slots for two physical SIM cards. Many newer phones accept one physical SIM and a second SIM as an eSIM (no physical card). Or in some phones, both the primary and secondary SIM are eSIM.
Phones with two physical SIM slots are uncommon in the U.S., but popular in certain other countries. Phones with a secondary eSIM are increasingly common.