Multiple programs at once

just to make sure, I just need a bit of code that will set a pin to go high for a set time and then low for a set time, for as long as the board is on, while still allowing other code to pass. This is what you were telling me to look at, right?

All I'm really trying to do is make a background 555 timer so on pin is always switching without having to manually do it so my sketch can focus on capturing events and running there code (watching for turn signal switches, key switches, throttle positioning and such)

Though now that i think about it, it might be considered an interrupt. where every [time var] then system suspends the current code at where ever it is, and switches the pin to the opposite and then continues about its way.

Sorry I am fairly at this coding language