The phrase "to thicken the cream" means to make the cream thicker or more dense.
Sentences with «to thicken the cream»
We had this with thickened cream and that really worked. (smittenkitchen.com)
The sauce will become thick as you whisk — simply add more water and keep whisking until it is the consistency of thickened cream. (arthurstreetkitchen.com)
Turn off the heat, stirring thickened cream so it doesn't burn. (remonline.com)