To “give tochahah to someone” means to reproach them for their behaviour, usually of a moral or religious nature. The word means, literally, to prove or compel. It is a commandment, from Leviticus 19:17, “You shall not hate your brother in your heart, but you shall surely reprove your neighbour and not bear sin on his account.”
The existence of this mitzvah reflects all Jews’ fundamental responsibility for one another. The bad behaviour of another Jew impacts on us directly.
However, the rabbis are well aware of how attractive it is to criticise others and how alienating that criticism can sometimes be. So they condition the circumstances when tochahah is appropriate. “Just as we are commanded to chastise those who would listen, so are we commanded not to chastise those who would not listen” (Talmud, Yevamot, 65b).
The Vilna Gaon (1720-97) writes that one should not reprove someone on their religious practice or lack of it if they profess not to be an observant Jew (Aruch Orach Chaim 608).
Also the way you give reproach is vitally important. The Talmud (Erachin 16b) understands the end of the verse, “don’t bear sin on his account” to be warning us not to sin in giving tochahah by humiliating your friend with your criticisms. Tochachah must be given out of love and after searching one’s conscience to make sure that you aren’t also guilty of the same thing you are about to reproach someone else for.