Middle C is the C closest to the middle of the piano, usually right in front of the maker's name. It's where most beginner music starts, and it's marked with a dot on the keyboard here.
To say exactly which C you mean, notes get a number: middle C is C4, the C below it is C3 and the one above is C5. The number goes up at every C, so the D just above middle C is D4 and the B just below it is B3. (Some keyboards call middle C "C3"; the key is the same.)
From one C to the next is an octave: twelve keys counting the black ones, or eight white keys counting both Cs. Two notes an octave apart have the same name and sound alike, one higher than the other. Play them together and they blend into one strong sound.
Practice
Three Cs
Play middle C, then the C an octave below it, then the C an octave above.
AC3SDDEFFGGHAJBKC4LD;E'FGABC5WETYUOP
Computer keys: ASD… for the white keys, WET… for the black keys, ZX move an octave.
More info
Tips
When you sit at a real piano, sit in front of middle C, far enough back that your elbows are a little in front of your body.
Playing the exercises
Use a MIDI keyboard (Chrome, Edge, Firefox or Opera), the computer keys (ASD… for white keys, WET… for black keys, ZX to move an octave), or tap the keys on the screen. A key lights up after a couple of wrong tries.
The answers
What's another name for middle C? C4. In the usual numbering middle C is C4. A few keyboard makers call it C3, but it's the same key.
Which note is just below middle C? B3. The number changes at C, so the B just below C4 is B3.
How many keys, black and white, are there from one C up to the next? Twelve. An octave is twelve steps: seven white keys and five black ones, then the next C.
What do two notes an octave apart have in common? The same name. They have the same letter and sound alike, one higher than the other.