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These two markets are based on the highest/lowest number of runs either side will score in an innings during the course of a Test Series.
Let's look at an illustration. At the start of an England v West Indies Test series, SportingIndex might reckon that the highest innings score would be 480 and therefore set their spread at 470-490. If you believed that the majority of pitches would be batsmen friendly, you would bet high at 490 for the stake of your choice, in this case perhaps £2 a run.
If England notched 530 in the first Test and this had proved to be the highest total of the series, this bet would have rewarded you with 40 times your stake: (530 - 490) x your stake = 40 x £2 = £80. However, had the batsmen struggled throughout the series and the highest total only been 440, you would have lost 50 times your stake. 440 - 490 x your stake = -50 x £2 = -£100.
The same principle applies for lowest innings score.