It gets weaker the more it's wounded. So at 13+ wounds suffered it is at its weakest.
The reason they write it down like this, on every damage table, is cuz it's sometimes possible to increase the wounds value of a model, for example using an artifact or spell. By writing X+ in the last row they guarantee the table remains correct no matter how much you'd manage to buff the model with extra wounds.
As an example if they just wrote 13 suffered wounds suffered instead of 13+, and you'd somehow manage to buff your bloodthrister to have 15 wounds he could be alive with 14 wounds suffered and the table wouldn't cover that situation. But by writing down 13+ that situation is still covered.