RCT Question

[bekib86]

What does this sentence mean in an RCT report?

Using a conventional 5% two sided test, with 80% power, the sample size required for prevalences of 50% and 60% in two groups was 388 per group (total 1164)

I do not understand this at all and I need to report whether the sample size was justified in my critique of the whole report.

Thanks