Weapon and Armor Proficiency
Source: Advanced Player's Guide, p. 54
Summoners are proficient with all simple weapons. Summoners are also proficient with light armor. A summoner can cast summoner spells while wearing light armor without incurring the normal arcane spell failure chance. Like any other arcane spellcaster, a summoner wearing medium or heavy armor, or using a shield, incurs a chance of arcane spell failure if the spell in question has a somatic component. A multiclass summoner still incurs the normal arcane spell failure chance for arcane spells received from other classes.