Many of us keep our hens confined to their run most of the day where they have access to their water fountains. So staying hydrated isn't an issue.
But when you let your hens out to free range, it's important that they still have access to fresh, cool drinking water. Especially, in the summer months when it's warm.
I fill up a ceramic bowl with cool water and put it in the shade on the patio for my girls. If it's really warm, you can even put in ice to keep the water cool. Put the bowl out where they can easily get to it. And be sure to dump it out after you put your hens back so you don't attract critters who want to share your hens water.