Backstory is everything that has happened to your character before your opening chapter, from childhood on up. ("When she turned eight years old, Susie's parents bought her a pink bicycle ..."; "Matthew had always had good luck with women ..."; "Amy went through a terrible divorce three years ago ...") Too much backstory is deadly to your pacing. Resist the urge to reveal more than absolutely necessary, and avoid doing it all in one chunk. Your reader only needs to know basic, crucial details relevant to the present story, and she's smarter than you think. She won't need to know every little detail to be able to follow your story.
The best way to reveal relevant backstory is to weave it throughout the present-day story. Dialogue with another character is a good way to do this, and still stay in the present tense. Some writers like to use a prologue to reveal an important past occurrence that will affect present events. Then, it's okay to use backstory in one chunk, because it's given in an immediately-occurring format - it's happening now (and then, in Chapter One, you fast-forward to Present Day). If you reveal only little bits of necessary backstory at a time, you'll avoid bogging down your pace with something that's already happened, and your reader won't get frustrated by plowing through past events. They want to know what's happening to your characters now, so they can follow along and share your hero's adventure.