To cook something over low heat means to prepare a dish by heating it slowly and gently in a pot or pan on a low flame, using minimal heat. Full definition
Add the diced tomatoes and continue cooking over low heat for about 10 minutes, stir and crush the tomatoes with a wooden spoon as they cook. (vansfoods.com)
Slowly bring to a gentle simmer, stirring often, then cook over low heat until the cheese is thoroughly melted. (vegkitchen.com)
Then add the sugar stir again and continue cooking over low heat for about 45 minutes. (itheecook.com)