Selection Rivals are those teams that you are essentially competing with for an NCAA at-large berth. The 'Net' value in the table is the difference between your NCAA tournament chances (at-large + conference auto-bid where relevant) when the team in question doesn't make the NCAA field (as an AL or auto-bid) and when they do.

As an example, let's say that for the first team in the list, the values are as follows...

In when In: 18%

In when Out: 34%

Net: 16%

That means that when the team in question makes the NCAA field (In), your chance of also being in the field is 18%. If the team in question is left out, then your chance jumps to 34% for a net of 16%.