The phrase "over simmering water" means to cook something in a pot of boiling water that is kept at a low temperature, allowing the food to cook slowly and gently. Full definition
Place bowl over simmering water and whisk until nicely thickened (about 10 minutes). (freecoconutrecipes.com)
Heat the butter, 4 ounces of the chocolate chips, and the unsweetened chocolate together in a medium bowl set over simmering water just until the chocolate melts. (pearltrees.com)
Melt the chocolate in the top of a double boiler over simmering water until smooth and melted, stirring occasionally. (guidosfreshmarketplace.com)