Polls are an attempt to find out what people think.
They work on the basis that a sample of 1,000 or so voters, if properly selected, can represent the views of the over 30 million voters in the whole country. If that seems like a small number, remember that it works like nibbling a cake - you don't have to eat the whole thing to know that it has got nuts and raisins in it. People taking part might not be a proper cross-section of the whole population. For example if a survey sample is skewed towards groups who are more Labour than the rest of the population then it will exaggerate Labour's support. People may give the answer that they think is socially right rather than what they truly believe.