The answer to this question depends on the age, fitness level, gender and other factors for each individual.

You should aim to increase the intensity of your exercise to get your heart rate up to 80-95% of your heart rate maximum (HRmax) during your working intervals.

You should then decrease the intensity to bring your heart rate back down to around 60-75% HRmax during your active rest intervals. One way to estimate your HRmax is to take 220 and subtract your age (i.e. a 25-year old has an HRmax of 195). 

Here is a chart from the American College of Sports Medicine that will help you differentiate between your own high-intensity and low-intensity intervals. The chart also provides information relating to the Rate of Perceived Exertion (RPE) which is a subjective measure of how difficult the workout feels for you.

Intensity

HRmax

RPE

Low (i.e. Walking)

57-63%

Very light to fairly light

Moderate (i.e. ET)

64-76%

Fairly light to somewhat hard

Vigorous (i.e. HIIT)

77-95%

Somewhat hard to very hard

All-Out (i.e. Sprinting)

>95%

Very hard