The polls are almost certainly somewhat wrong, but not by much. (Some are politically biased as well, but that’s a separate issue and the biases are often in favor of the GOP.) All polls use statistics to quantify how much wrong they could be. When a poll reports “Trump’s support is at 37% plus or minus 2.0 percentage points” (as did the recent Reuters/Ipsos poll), then that true value is between 39%–35% with the most likely value being 37%. Some polls or news reports don’t report the error range, which is unfortunate and deprives you of useful information.
If you want to know how that error value is calculated and what affects it, you’ll want to study statistics and understand how that works in some detail. More detail than I can answer in this question.