These questions are about one of my devices, a Leviton DZ6HD wall dimmer.
Here is the configuration page for it.
CONFIGURE- What does this do? When I click it just sends a warning to the log.
FLASH- Seems to flash the light connected to the module. I suppose you can use this for IDing a device. Don't really see any other utility for this unless I'm missing something.
OFF/ON- These two turn the light ON or OFF. Okay, got it.
REFRESH- Clicking this does nothing. What's it supposed to do?
SET LEVEL- Seems to set the dim level maximum for the device. I thought it would allow me to set the dim level from Hubitat without changing anything in the device, just like ON/OFF.
STARTLEVELCHANGE/STOPLEVELCHANGE- Can anyone explain what these do?
Those are commands, not so much configuration (though most drivers do expose some in the "Preferences" section just below that area in your screenshot). Most are a way to manually control the device, as opposed to using an app/automation, dashboard, etc., so can be useful for testing or experimentation (not really the best UI for day-to-day control, though it also technically could work for that).
"Configure" is a bit of a special command; this sends configuration, which often might be related to the preferences you've chosen in the section just below this, to the device. Sometimes it also sends configuration to the device that affects if/how the physical device reports things to the hub. Normally, this is run automatically upon pairing and with a "Save Preferences," so you usually don't need to do it yourself, but if you switch drivers or are having problems that seem related to any of these things, it could be a good idea.
"Refresh" is also a bit special; it will just ask the device for its current states (like dim level and on/off). most devices, especially new-ish ones like yours, report this information back on their own, so this is again not normally something you'd need to run, but it can be useful if the hub ever gets "out of sync" for some reason (e.g., you were updating/rebooting your hub when you turned it on/off and want the new state without having to change the device again).
"On" and "off" are pretty self-explanatory, as you've figured out; "Set Level" sets the dim level/brightness on the device. So 50%, as you've put in, should be the same as if you went to the wall and dimmed it down halfway. "Start Level Change" and "Stop Level Change" begin raising (if you choose "up") or lowering (if you choose "down") this same level until you either reach the max/min or run a "Stop Level Change." It's a pretty neat way to get smooth dimming from a hub automation, like if you have button device you want to control these lights. Not all dimmers support these commands, but if you see it on the device page like you do, it's likely yours does.