p is the sample proportion of people who reported earning money by selling something online (p = 914 / 4579 = 0.2)
n is the sample size (n = 4579)
So, the standard error for these data is:
SE = √(0.2 * (1 - 0.2) / 4579) = 0.0134
The interpretation of the standard error is that it gives us an idea of the precision of our estimate of the population proportion. Specifically, it tells us the average amount that our estimate of the population proportion would differ from the true population proportion if we took many samples.
In this case, the standard error of 0.0134 means that if we took many random samples of 4579 American adults and calculated the proportion of people who reported earning money by selling something online in each sample, the average difference between these sample proportions and the true population proportion would be approximately 0.0134.