A water level gauge allows you to see how much water is left in the pot.
When the meter reaches zero, there is still an amount of water present in the substrate.
This is why the plant will tolerate several days without watering. For cacti on hydroponics this may even take a few weeks.
Note that some plants have bigger water needs and then it is best not to wait too long before watering again.
Do you have to wait a few days before watering?
No, you don’t have to. You may water when the meter reads zero.
But you have to rinse regularly:
What you should do is rinse the pot and clay pellets thoroughly with clear water every 3 to 6 months.
Remove the growing pot from the outer container and rinse the pot and granules, with plant and all, under the tap or shower.
Wash the outer pot thoroughly with clear water (no soap!). The clay pellets that stand between the growing pot and outer pot may be rinsed as well and reused.
This way you avoid having a non-equilibrated nutrient solution over time as some nutrients remain in the pot between waterings.