First Principles Question

Hello all

I am taking an introductory calculus course at the high school level currently and had a question about first principles

basically when one finds the derivative dy/dx based on first principles, what does the answer represent??

For example, the function f(t) = -4.9t^2 + 16t + 1

when you find the derivative of this based on first principles, you get

-9.8t + 16

what can i do with the -9.8t + 16, can i use it to find instant rate of change at any value of t or no?

thanks :)