Dishwasher won't drain

Ok the dishwasher seemes to run fine thru all the cycles… but after it doesn’t drain. leaves about 2" water in the bottom. We just bought it off craigslist and got it installed. What do yall think it might be? We leave about 3-4 cups of water start a cycle and stop real quick seems to start to drain but then water just comes back up the drain.