Summon Resist increases all of your minions' abilities to survive elemental attacks. It is not advisable to spend more than one point into this skill however, as the returns are fairly weak. Getting some items with + to skills is generally advised.
Powerful heat, freezing cold and intense electrical attacks can take a serious toll on the bound servants of any Necromancer. By developing this skill, a Necromancer can permeate his summoned minions with a protective energy that strengthens their protection to the natural and mystical elements alike.
↑Diablo II Manual, page 77. Blizzard Entertainment, 2000.