As a Mnemosyne beginner, I find the tooltips in the desktop client very helpful. I suggest the same tooltips are added to the Android client on long pressing each grading digit.

For clarity, the tooltips I'm talking about are:
"You don't remember this card yet."
"Like '0', but it's getting more familiar. Show it less often."
...
"You've memorized this card now, and should remember it in 2 days."