The particle -(이)라도 marks the thing you look at as a fallback or a bare minimum rather than the real goal, and 보다 is seeing or watching. In these lines it covers both settling for less (안 되면 하나라도 보고 — if not, we'll at least see one) and the 'even once' sense (단 한 번이라도 봤냐? — have you seen it even once?). A near neighbour is -도 보다, which just adds 'saw that too'; -라도 보다 concedes that this is the lesser option.