"To simmer the mixture" means that you need to heat the ingredients on low heat, just below boiling point, until small bubbles appear and the liquid becomes hot. Full definition
Continue simmering the mixture for 40 minutes, or until tender. (cookingchanneltv.com)
Gently simmer the mixture, uncovered, until the ginger is tender, about 1/2 hour. (fieryfoodscentral.com)
When the caramel is ready, slowly whisk in the warm cream and continue simmering the mixture until it is smooth, another 2 to 3 minutes. (katherineinbrooklyn.com)