What is backtracking?

def subsets(nums):
    result, path = [], []
    def backtrack(start):
        result.append(path[:])           # record a copy
        for i in range(start, len(nums)):
            path.append(nums[i])         # choose
            backtrack(i + 1)             # explore
            path.pop()                   # un-choose
    backtrack(0)
    return result


# --- demo ---
print(subsets([1, 2, 3]))   # [[], [1], [1,2], [1,2,3], [1,3], [2], [2,3], [3]]