Sankranti means to go from one place to another place (to change direction). It also means one meets another. The time when the sun changes direction from one constellation (of the zodiac) to another is known as Sankranti.
It is celebrated in mid-winter and marks the transition of the sun from the Sagittarius to Capricorn during the winter solstice in the Northern Hemisphere. Whereas the rest of Hindu dates are calculated according to their Lunar-solar calendar, so dates shift in reference to our Western Gregorian Calendar, Sankranti is Solar event, so in this era it remains at the same date. Although it is shifting, and over 9000 years it will be somewhere in June.
People take dips in rivers and worship the Sun God especially in the holy Ganges river. The dip is said to purify the self and bestow "punya". Special puja is offered as a thanksgiving for good harvest.