Sentry Mode is a security feature that arrived via an over-the-air software update back in early 2019, in response to a rash of break-ins on Model 3’s in Southern California (the little triangle window behind the passenger door does not trigger the alarm when broken, and people were smashing that to then fold the seat down to see if anything was worth stealing in the trunk). It expands on DashCam, which uses the same cameras as AutoPilot. All Model 3 and Model Y vehicles support Sentry Mode. All Model S and Model X cars built after 2017 as well as any that have been upgraded to MCU > 2 should be Sentry Mode capable. You will see the option in your car’s settings and in your smartphone application.
The other requirement for Sentry Mode is a USB storage device plugged in to the front USB ports (newer cars, as of 2021, actually have a USB port in the glove compartment, for added security). The drive needs to be formatted in either FAT32 or ext4, and must have a directory called “TeslaCam” at its root. Preparing a new USB storage device for Sentry use can be done within the car with recent software updates.
Once those are taken care of, you just enable Sentry Mode from your car or smartphone app. It may be a good idea to turn off Sentry Mode at home and maybe work, as even with recent software versions (including 2021.4.x) there may be a lot of false positive noise, as Sentry Mode reacts to any perceived nearby motion, including heavy rain. Another reason for periodically disabling Sentry Mode is to minimize the chances for writes to fail and the drive to become corrupted.
The larger the storage device the better, but 128G should be absolute minimum. Note that SSD drives do work, but that is not the best use for that technology as this will be heavily skewed towards writes and not reads. As such, SSD drives may fail sooner than expected in those environments (they should still last longer than you have the car and others have said they last longer than most USB thumb drives, but portions may fail sooner than expected). You also want one with fast write speeds; if the device is too slow, you will get a warning in the car and videos will be degraded if they work at all. One described as for “gaming” should be more than adequate.