1.Lit. to cook the fat out of something. Polly rendered the chicken fat down to a bit of golden grease that she would use in cooking a special dish. Jane rendered down the fat for use later. The cook rendered it down.
2.Fig. to reduce or simplify something to its essentials. Let's render this problem down to the considerations that are important to us. Can't we render down this matter into its essentials? Not all of this is important. Let's render it down.