[Solved] Could you please explain this piece of code?

This is a very concise list comprehension form, which is equivalent to the following:

res = []
for value in db.cursor.fetchall():
    pairs = []
    for index, column in enumerate(value):
        pairs.append((columns[index][0], column))
    d = dict(pairs)
    res.append(d)

The res list is equivalent to what you wrote above.