It is the promotion by institutions of the inherent superiority of heterosexuality and the assumption that everyone is and must be heterosexual.  Like racism, sexism, and other forms of prejudice and discrimination, heterosexism awards privilege to members of the group with more power (heterosexuals).

This is the first time I have heard this term, but growing up in the "Bible Belt" makes one see this point of view over and over! I grew up thinking something was wrong with me because I did not feel the same way as I thought everyone else in the world did. I did not know anyone who was gay. I thought gay people were only in movies. It made me feel very confused and that I was alone.

I always wished that I had someone who I could have shared my feelings with growing up. It would have been so much easier and less sad.