What does 1 inch/week of water mean? This is a very unclear statement. What does it mean? And do I give this amount throughout the week or only once?

It's a general guideline. Probably more useful for trying to grow a lawn than installing new plants in a non-mowed area. I don't pay any attention to those kinds of measurements & water plants if they are thirsty, don't water if they're not.

If your rainfall is about 1" total for the week, doesn't have to be all at once, they're saying you likely don't need to water the plant anymore. But as purpleinopp said, it's really best to water when it's thirsty. Many things can affect how much of that rain the plant actually gets... partially under an eave, thick dense foliage that might hinder rain actually getting to the roots, etc. And that's only for in ground plants. If you plant in containers, you'll most likely have to water them every day, sometimes twice a day.

A straight sided container will measure the inches. So if you set up a sprinkler for an hour or two, with a straight bucket, can, etc in the watered area, when the water in the bucket measures an inch deep, that is watering an inch.

We use a weather app and use reports from a nearby station to estimate what we are getting naturally.

One inch of rainfall/irrigation amounts to .62 gallons per square foot. Or 62 gallons
per 10' X 10' planting area. If your hose is squirting 5 gallons per minute, you would give that much to your area after 13 minutes of watering (try to get it all on evenly!).

Personally, I think "1 inch per week" is a bare survival amount, in very mild weather conditions. I easily put 2 or 3 times that amount on my vegetable gardens in summer, in the not-so-torrid climate we have in Seattle.

It also depends on how that inch is divided up. I live in an arid, high climate with plenty of wind. If I watered an inch on Monday and didn't water again for a week everything would be dead. I'm better off watering a fraction of an inch every day or two.

And we only get ~13" of rain a year so I ignore what rain we do get when I'm considering watering. Any rain is just frosting on the cake, its not enough to keep anything alive.

Henderman said: It also depends on how that inch is divided up. I live in an arid, high climate with plenty of wind. If I watered an inch on Monday and didn't water again for a week everything would be dead. I'm better off watering a fraction of an inch every day or two.

And we only get ~13" of rain a year so I ignore what rain we do get when I'm considering watering. Any rain is just frosting on the cake, its not enough to keep anything alive.

That is all super interesting! Thank you! Is it hard to overwater plants that are in the ground? I'm so worried about overwatering.....

It is hard to overwater in the ground. It's a giant sponge.

An inch of water a week is good for most heavy soil but where the soil has high sand content, that may fall short.
I use over head sprinklers and generally set them for one hour, plus or minus, on a vegetable garden if rain has been absent.
Unless you live where you have standing water, often, you will not over water.