What are metal pipes attached to the front of buildings between the first and second floors?

They are used for holding flags or banners. Though not much in use today, except in public buildings where they sport Czech flags on national holidays like October 28, under communism, it was mandatory for all apartment buildings to be decorated with Soviet flags and red banners on designated holidays. For example, on May 1 or Great October Socialist Revolution Day (November 7) not only did all buildings have to fly Soviet and Czech flags – notice that in most cases there are holders for two flags – but that every window in the building had to sport a banner, flag, or slogan. Prizes were awarded to apartment buildings with the highest participation rates and punishments meted out to buildings and individuals who neglected their civic duty.