Ah, backstory. That stuff that happens before your story starts. The reason why your character is so messed up and why he or she is in these circumstances in the first place.
I like backstory--that is, I seem to spin backstory more easily than I do plot, which is a problem. Your character's childhood, family, reason why he/she is afraid of spiders, where they grew up, what kind of education they received...
Unless any of this is the main plot of your story, then it is backstory--and should be revealed sparingly.
For example, in my book, my main character Miles has quite a lot of backstory. I'm not sure if there's too much of it for a reader in the text, but most of it, I think, are issues that carry over into the plot. His daughters, for instance, are characters in the story. His business is an ongoing entity. His feelings of distance from his family also carry over into the novel itself.
I still wonder if I may have laid on too much backstory in the story itself. There's even backstory of the backstory--the reader kind of learns that Miles's family is noble, but maybe hasn't been noble all that long ("not all that long" being, like, a hundred and forty years) and Miles's mother, when referring to how "old" the family was, only meant her own family--because they were prominent before the Keegans "got out of Irish bogs."
Honestly, writing about people with noble families is very strange sometimes. They start mentioning ancestors and they measure their title in generations and they talk about their "connections" and which ancestor did what under which monarch...
And I sit here, being very working class, and think, "Where did you weirdos come from?"
Anyway, back on topic...Is it strictly necessary to figuring out the plot? No. Does it lend a little flavor into Miles's childhood or the attitudes he was raised with? Yeah.
I'm outlining my NaNo project right now, trying to balance one idea (a love story) with another one (sort of a family saga). They both have backstory involved. I still have over a month to figure out how this thing is going to go. I know for sure that it's going to be contemporary---and largely set in New York City. I'm going easy on myself this November ;-)
The backstory of the Crawley family in Downton Abbey is, basically, that an American heiress came to England in the 1880s, was presented in London, met Robert who was then Viscount Downton, and married him, thus saving Downton Abbey and its estate.
And then they didn't have a son.
When does all of this get mentioned?
Throughout the entire first season, in drips and drabs. Of course, with TV, you can hear that Lady Grantham (never Lady Cora) is American, that there are obviously three daughters in the house and no son, etc.
For more on backstory:
Baby Got Backstory
How to Weave Backstory Seamlessly Into Your Novel
Is Backstory Clogging Your Narrative Flow?
I Wasn't Born Yesterday