If something is rustproofed, it has been treated so that it will not get rusty.
Rustproofed things are usually thought of as lasting a long time and being good quality.
Examples:
The rustproofed car never gets rusty.
I want to buy a rustproofed bike.
The rustproofed metal is very strong.
My dad's old car is not rustproofed, so it gets rusty.
The rustproofed ship sailed the ocean for many years without getting rusty.
The rustproofed bridge is very famous for never getting rusty.