Water was leaking behind the diswasher. I pulled it out and drained water from the sink to see where the leak was coming from. Water was pouring out from behind the sheetrock. I cut some of the sheetrock away and I see a vertical black pipe -not attached to anything coming out of the concrete slab. This is where the water is flowing over.

What the heck is that pipe? What is it's purpose? Why would it be overflowing?
I have a similar pipe like this in the laundry room that was filling up with water when the
Snow was melting last winter. I filled it up with caulk. Bad move?
Help please.
Thanks, Trish

If you had removed the drywall about 18" off the floor you would have found a sanitary tee that picks up your sink through a horzontal drain line. Something's open in that horizontal line in the wall. It also sounds like the drain was clogged under the slab, this in the reason you have water backing up. Before you do anything I would have the kitchen sink drain snaked out from the vent. After that's cleared up you can check to see if the horizontal line in the wall's OK.

I have a similar pipe like this in the laundry room that was filling up with water when the
Snow was melting last winter. I filled it up with caulk. Bad move?
Sounds like you plugged up the washer stand pipe. This is OK unless you have a washer that you wish to use. The pipe was filling up for the same reason that the kitchen overflowed. A blockage in the drain line. Your drains need attention. Have them snaked and cleared. Good luck, Tom