If you support someone else, you would say "I have your back" or "I've got your back."
It's a reference to military/police tactics. If someone is "watching your back" it means they are making sure that you will not be attacked/shot from behind while you are attempting to move.
By analogy, someone who has your back is watching out for you, keeping you safe while you try to accomplish your mission. Supporting you in what you do.
You will also see "got your 6" with "6" being a reference to the 6 o'clock position on a clock, which is directly behind you if 12 is in front of you.
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