what does it mean to say alpha is set at 0.5

When setting up a statistical test, two hypotheses are formed: a null hypothesis and an alternate hypothesis. The null hypothesis tests the probability that the sample mean (taken from sample data) is very likely drawn from the population mean. If there is a low probability that the sample mean has been drawn from the population mean, the null will be rejected and the alternate will be accepted. The significance level is the probability at which the null will be rejected and the alternate accepted. The significance level is usually denoted by the Greek symbol α (lowercase alpha). For example, if a significance level is set at 0.05, there is a 5% probability that the null will be rejected. (This is a very common level used in statistics.)

I hope this brief explanation will help.