Shto-to is more specific than shto-nibud'.

"Ty mne skazal shto-to sevodnya, no ya zabyl, shto eto bylo" - You told me something today, but I forgot what it was. (I think this is more Koe-shto)

Shto-nibud on the other hand, translates to 'something' that, in reality, means 'anything', so:

"Skazhi shto-nibud" would mean "Say something!". You can see that that 'something' in that sentence actually means 'anything.