Kumbha Sankranti marks the beginning of the eleventh month in Hindu Solar Calendar. All twelve Sankrantis in the year are highly auspicious for Dan-Punya activities. Only certain time duration before or after each Sankranti moment is considered auspicious for Sankranti related activities.
For Kumbha Sankranti sixteen Ghatis before the Sankranti moment are considered Shubh and the time window of sixteen Ghati before Sankranti to Sankranti is taken for all Dan-Punya activities.
A mela is simply a festival where large crowds congregate, in principle of a religious nature though the term is also applied more loosely. A kumbha is a pot or jar or pitcher, i.c. the one in which the gods had collected the immortality elixir or amrta. When they were fighting over it, they spilled four drops which fell down on earth at the four places where the Kumbha Mela is now held. But Kumbha is also the name of the Zodiac sign of Aquarius, which happens to have the same amrta symbolism of life-giving liquid poured down from heaven on all of us.
During Kumbha Sankranti offering to cows are considered highly auspicious. Also taking bath in Ganges especially in Triveni where Ganga and Yamuna converge is considered highly auspicious.