Sinat chinam means groundless hatred. (The verb soneh means to hate, as in the command lo tisnah at ahicha blevavecha, do not hate your brother in your heart, Leviticus19:17)
Chinam comes from chen, grace. Sinat chinam is therefore hatred that is gratis. It refers to the internecine strife which is unfortunately too common in Jewish communities, whether between Reform and Orthodox, Ashkenazim and Sephardim, the rabbi and the chazan, the president of the shul and the board.
You could charitably ascribe its existence to the high-stakes decisions that Jewish communities have had to make, or to a persecuted people internalising the hatred directed at them, and then projecting it against other groups of Jews. Either way, there is clearly too much of it about.
The Talmud already knew of the phenomenon and its destructive effect on Jewish life. Yoma 9b records that the First Temple was burned down because of idol worship, sexual immorality and bloodshed. At the time of the Second Temples destruction, the Jews were, on the other hand, pious but the Temple was lost because sinat chinam, groundless hatred, was endemic to Jewish national life.
From this the Talmud infers that groundless hatred is as grave as idol worship, sexual immorality and bloodshed put together.
Rabbi Abraham Isaac Kook, first Chief Rabbi of Israel, famously wrote that if the Second Temple was destroyed and the people scattered through sinat chinam, then the Temple will be rebuilt and the people gathered together again though ahavat chinam, causeless love.