An ally  is a person who is not a part of a minority community, but supports them and their cause. Such people use their privilege to make the world a safer space for people of this community, and to give them space to speak up and exist in larger spaces in the society.

You can be an ally to the queer community by respecting their cause, educating yourself and people about the community, giving them the space to express themselves, and being  non judgmental.

Being an ally also includes making a  conscious effort to acknowledge your privilege, and use it to help the minority community in question.