# What does f(x)=3 mean?

It means that you have a function that every time you input a value of $x$ gives you $3$.
If $x = 45$ then $y = 3$...if $x = - 1.234$ then, again, $y = 3$ and so on!
It is like to take a bus that charges a fixed fee of $3 regardless of the duration of the journey...you can say "I want to go to this or that stop (this is the $x$)" it doesn't matter...you'll alway pay$ 3 (that is the $y$).