Who is the authority on Gallop polls ?

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When watching the news,one can't help but notice news anchors instill worry/hope based on the latest figures released by some newspaper that the Presidents approval ratings has gone up or down. what i don't understand is how could they conclude that this is the sentiment of the entire population, it is based on a partial sample from the population who are willing to vote, and the fact its all done online makes it even less credible.At this day and age were most nationwide presidential elections are contested, how could a gallop poll by US today or other publication could be accepted as representing the entire population?