Selection pressure is basically an external pressure that an organism faces to be selected (i.e. to not die) and therefore continue to reproduce and spread its superior genes.
So if the temperature (an abiotic factor) increases in the Arctic, the raised temperature acts as a selection pressure (it's an environmental pressure that will lead to selection of some organisms over others). In this case, rabbits with summer fur will survive whereas those that don't change their winter coat to a summer one won't.