MDI is Manual Data Input and it is entering a move from the MDI line.
Say for example you zero your Y axis then you type something like G0Y10 then pres your keyboards enter key, the axis will rapid 10 units in a positive direction.
Similarly if you type G1Y10F10 the axis will move 10 units but this time at a feed rate of 10 units per minute (assuming you are in feed per minute G94) mode)
Hood