I need to learn what?

人海 (i.e. 眾) is my guess. This is like 海選.

My understanding is that 海 means "vast" here. Here is a story on the origin of the term -> “海报”探源

上海方言中又习惯于以“海”来形容“大”或“神气”，如称气量大、酒量大叫“海量”，大碗叫“海碗”，说大话叫“海话”、“海口”，称有气派、神气为“海会”（如形容某人衣着神气，会说“身浪穿得海会来”）。因此，当面对此类前所未见、又大又神气的新型戏报，上海人不由得发出感叹：海会得不得了的戏报！略而言之，就成“海报”。

Thanks skylee, that makes a bit more sense.