Help me guys...

AWOL: If the time in millseconds now, minus the time at some previous juncture is greater than a certain value, then perform some action.

but before you do, save the current time so that you know when the action was carried out.

RobinAnd000:

if(currentMillis - previousMillis > interval) {

previousMillis = currentMillis;

This is only an incomplete part of a piece of code. There is no closing }.

The first line checks whether the time difference is greater than the interval
The second lines updates the value in previousMillis so it can be used for the next time check.

What is also missing is the other code that should happen when the interval has elapsed - such as turning on/off an LED, for example. There is not much point doing timing for its own sake.

if(currentMillis - previousMillis > interval) {
    previousMillis = currentMillis;   
    digitalWrite(ledPIn, ! digitalRead(ledPin));
}

…R