What do shifts in polling numbers actually measure?

According to Andrew Gelman, such shifts have little to do with actual changes in voter preferences between candidates, and much more to do with temporary shifts, largely driven by the news cycle, in the willingness of supporters of particular candidates to take part in polls:

Why, then, do the polls swing so much? This can mostly be explained by differential nonresponse to pollsters: Clinton goes up when more Democrats answer a survey, and Trump goes up when Democrats are less likely to respond. As Lauderdale and Rivers put it, “when things are going badly for a candidate, their supporters tend to stop participating in polls.” This is how new events can have big effects on the polls even if they aren’t changing many vote intentions.

For example, after the FBI letter on Clinton a week or so ago, I predicted that Clinton would fall in the polls — not because she was going to lose many votes but because the news would pump up a lot of Trump supporters who would then become more enthusiastic about the election and respond to surveys. The preceding weeks had been full of bad news for the Republican candidate, hence his supporters were dejected and not participating in polls. During that period, I and others suspected that Clinton’s lead in the polls had been exaggerating her strength among the electorate.

Gelman notes that this methodological problem is exacerbated by the sharp decline over time in the percentage of people who are willing to take part in voter surveys, which is now often just a few percent of the initial contact pool:

 

Differential nonresponse is a bigger deal now than it used to be, for two reasons. Survey response rates are lower. Not too many decades ago, quality polls had response rates over 50 percent; now a survey is lucky to get 10 percent participation. As a result, responding to surveys is much more optional, and we’d expect differential nonresponse to be a bigger deal. At the same time, the electorate is more polarized, and fewer people change their minds during the campaign. Thus, compared with previous decades, the “signal” of actual swings is lower and the “noise” of nonresponse is higher, and we need to be concerned about this source of bias more than ever before.

The upshot of this I suppose is that horse race coverage of elections, and especially presidential elections, is to a significant degree producing the “news” that it’s purportedly reporting.