A dishwasher with standing water at the bottom after a cycle is almost always one of three things.
First, a clogged drain. Food debris, broken glass, and label residue collect at the sump filter. Most modern dishwashers have a removable filter assembly — pull it out, clean it under running water, and reinstall. This fixes a surprising number of no-drain complaints.
Second, a blocked drain hose or a kinked connection at the garbage disposal. If the disposal was recently installed, confirm the installer removed the knockout plug on the disposal's dishwasher-inlet nipple — leaving that plug in place blocks drainage completely. Run the disposal to clear any debris sitting in the disposal's discharge tube; the dishwasher drain shares that plumbing.
Third, a failed drain-pump motor. Listen for a humming or silent pump during the drain phase of the cycle. A failed pump typically runs $250 to $400 to replace, and the part is in stock for most major brands.
Fourth, less common but worth mentioning: a clogged air-gap fitting at the sink top or a failed check valve. Both are fast repairs.