Basically, it should mean "a function which can attain non-negative values only".

Anyway, could you quote the whole sentence in which the term occurs?

Thank you Kowalik for quick answer!

Please provide the context where this is mentioned.

Sobhan Sarkar, I agree the context in which the term is used is very important. My answer was just a guess.

Non negative function is a function when it attain non negative values only. In detail provide the problem where it is mentioned.

Thanks for all answers from RG Colleagues

Non negative functions takes non negative values but depends on the situation where it use.