These cause readings to be spread about the true value due to results varying in an unpredictable way from one measurement to the next.
Random errors are present when any measurement is made and cannot be corrected. The effect of random errors can be reduced by making more measurements and then calculating a new mean.
Calculating a mean works because the variation from the true value is just as likely to be above the true value as below it. By taking a mean the errors above the true value will tend to even out the values below the true value. This is why an increased sample size will increase the accuracy of the mean.