What does mean reversible inhibitor of histamine at the histamine H2-receptors?

I was diagnosed with GERD (gastro esophageal reflux disease) and was prescribed with Ranitidine. Somewhere I found information that Ranitidine is reversible inhibitor of histamine at the histamine H2-receptors. What does mean reversible inhibitor of histamine at the histamine H2-receptors? Maybe I do not nee to know that but I am very curious man.