During the winter solstice in the Northern Hemisphere, the Sun is directly overhead at the Tropic of Capricorn.
The Northern Hemisphere’s winter solstice occurs at the December solstice, when Earth’s Southern Hemisphere is tilted most toward the Sun. At that moment, the Sun’s subsolar point—the place where sunlight strikes most directly—is near 23.4° south latitude, the position of the Tropic of Capricorn.
This is why the Tropic of Capricorn marks the southernmost latitude where the Sun can appear directly overhead. The opposite event occurs at the June solstice: the subsolar point reaches the Tropic of Cancer, at about 23.4° north latitude. The Equator lies between the two tropics, so the Sun is directly overhead there only around the equinoxes.
A common mix-up is to associate “winter” with the hemisphere experiencing the solstice. Seasons are opposite across the Equator: while the Northern Hemisphere has winter, the Southern Hemisphere is beginning summer. The line’s name refers to the constellation Capricornus used in ancient sky maps, though precession means the Sun no longer appears there at the December solstice.