GLAAD on the definition of a straight ally:

LGBT people are our mothers, fathers, sons, daughters, sisters, brothers, aunts, uncles and cousins. This is a fact and it isn't going away. You have the opportunity to be an ally and a friend at home, school, church and work. A straight ally can merely be someone who is supportive and accepts the LGBT person, or a straight ally can be someone who personally advocates for equal rights and fair treatment.

Allies are some of the most effective and powerful voices of the LGBT movement. Not only do allies help people in the coming-out process, they also help others understand the importance of equality, fairness, acceptance and mutual respect.