A chumrah is a voluntarily assumed restriction more stringent than what is required by Jewish law. For example, drinking only chalav Yisrael, supervised milk, is a chumrah according to most authorities. In a country like Britain, with high standards of food regulation, the chances of milk being tampered with are negligible.
Chumrah comes from the word chamur, meaning heavy, grave or serious. Keeping chumrahs can be an admirable part of saintly religious behaviour. However, there are rabbinic sources that caution against adopting them. "Are the Torah's prohibitions not enough to you, that you have come to create new prohibitions for yourself?" asks the Jerusalem Talmud (Nedarim 9:1).
In his classical ethical work, "Mesillat Yesharim," "The Path of the Just," Rabbi Moshe Chaim Luzzatto advocates stringency beyond what the Torah demands only after one has achieved scrupulous observance of all that is required - a lifetime's work for most people.
Furthermore, chumrah is meant to be a level of religious observance that exceptional individuals may take upon themselves, rather than one which is adopted by a whole community as if it is a norm. And one may adopt chumrahs in tzedakah, giving charity, or lashon hara, harmful gossip, just as much as in the areas of kashrut and Shabbat.