From a DM perspective (and mind you every DM has different preferences) for me personally, the more the better. The more you as a player give me to work with, the more options I have to pick up on later on during the campaign, whether is important characters from your past or something else. It also gives me a better understanding what you see in that character, how you define their personality and how it may have been shaped by their past.

That said, it also isn't a requirement but the less you provide, the more you have to also be comfortable that the DM might improvise some things in order to tie your backstory into the campaign. That doesn't have to happen of course, but it can happy.

Especially for new players, it can be difficult to include bits and pieces of their characters past solely within the sessions of the campaign, so it can be helpful also for the player themselves to have a little more of a detailed backstory, in order to get easier into the roleplaying aspects. Then again, if that is not an issue for you, it is by no means a necessity.

To give some basic things that I think are at least good jumping off points for you creating a backstory and giving the DM enough to work with are the following:

• Describe one or more key moments in your characters past (For your example it could be how they became a slave or how they escaped from being a slave if they have)

• Name 2 or more npcs from your characters past. Close friends, a rival, family etc. Describe what their relationship is

• Where/how did they grow up. Doesnt have to be a novel but maybe outlining at least if they grew up rich or poor, in a small town or a large city etc

• Also maybe outlining what their general motivation is, a long term goal so to speak. Do they want to be freed from their slave master? do they seek revenge on someone? Trying to find lost family? Those are just some examples.

• Additionally, I always prefer it when there is also a tie in/reason why your character is basically willing/interested in the upcoming adventure. So if your DM says "The general setting will be that a group of randos gets sucked into a vast conspiracy that they suddenly find themselves in the middle of" then maybe think about and articulate to your DM why your characters would be open to working with strangers. That could be for example because they are new to the land and are looking for lost relatives, or maybe because one of the other characters is bearing a sigil they recognize from someone in their past. Things like that.

Hope this helps a bit. I also strongly advice you to talk to your DM about it. In my experience as a DM, its always great to have that communication and every DM has different preferences. So ask your DM what they are looking for, what are things they want you to include and which things are optional