It means:
What length can you measure in only one attempt?

Exemple, you have a “clean” ruler (20cm, with all the measures between it start and end) and a line to measure (> 20cm)You can measure the “first” 20cm “put” your finger, and start checking the length from your finger, add the two values. So that’s 2 attempts at least.

From your maths problems, the simplest values to find are: (0,) 2, 5, 9, 11 and 12.
But in fact, you can also measure 3 (starting point at 2, end at 5), 4 (5;9), etc.