函数是数学里的术语，功能是指作用、用处、指东西能用来做什么，它们两个是完全不一样的意思

I think that the reason why you are confused by 功能 and 函数 is that they are both called "function" in English. But as you can see in English dictionaries, there are several definitions of the word "function", two of which are: 1 the purpose that something has, or the job that someone or something does The church fulfils a valuable social function. 2 [C usually singular] technical a quantity or quality whose value changes according to another quantity or quality that is related to it The degree of drought is largely a function of temperature and drainage. The first definition is the same as that of 功能, while the second is the same as that of 函数. 函数 can be used in computer science as well as in mathematics. In computer science, it means a sequence of program instructions that perform a specific task, packaged as a unit. And the English for it is also "function".