Knowing how easily water infiltrates into your soil helps you understand how well it is soaking up rain. You want water to be stored in the soil profile and not to run off into water courses.
If your infiltration rate is quick, it indicates your top soil structure is good. The top soil has stable aggregates, around which water can percolate downwards into the soil profile.
If your infiltration rate is slow, it indicates your top soil structure is poor. The top soil has unstable aggregates and is compact, so it blocks water from percolating downwards. Instead water runs off taking soil and nutrients with it!
The testing area should not be saturated so if it has rained heavily wait until the area is a bit drier.
Here are instructions on how to make an infiltration rate tube.