Supremacy is the margin, in goals, one team will beat another by.

Example:

Manchester United v Arsenal – Man Utd/Arsenal: 0.6 – 0.8

This quote says United will beat Arsenal by a margin of between 0.6 and 0.8 goals

If you think it will be more than 0.8, you go higher (buy).
If you think it will be less 0.6, you go lower (sell).

Manchester United beat Arsenal by 2 goals.

If you had bought at 0.8, you would win your stake multiplied by 1.2 (2 – 0.8)

If you had sold at 0.6, you would lose your stake multiplied by 1.4 (2 – 0.6)

Arsenal beat Manchester United by 2 goals

If you had bought at 0.8, you would lose your stake multiplied by 2.8 (-2 – 0.8)

If you had sold at 0.6, you would win your stake multiplied by 2.6 (2 + 0.6)