The "T" rating stands for Teen, assigned by the Entertainment Software Rating Board (ESRB). It indicates content generally suitable for ages 13 and older, but parents should understand the nuances.
Other regions use different systems: PEGI (Europe), CERO (Japan), USK (Germany), GRAC (Korea), and ClassInd (Brazil). A "T" rating roughly aligns with PEGI 12 or 16 depending on content.
The T rating is a useful starting point, but the Content Descriptors on the back of the box (or the ESRB's online summary) tell you what your teen will actually encounter. Use them to make informed decisions for your family.