An interval that includes the end points.
Example:
The interval 0 ≤ x ≤ 20 is all the numbers between 0 and 20, including 0 and 20
Could also be written [0,20]
Imagine a flight allows luggage "no more than 20 kg", so 20 kg is allowed, and 0 kg is allowed as that is simply no luggage: so: 0 ≤ luggage ≤ 20 kg
See: Open Interval