Help on codeforces

what r pretest on codeforces wht to do if u get wa on pretest ?

In simple terms,
Suppose there is a problem and it has total 50 test cases. But during the round, your solution will be tested for around 5-10 test cases. Now if you get AC in these, it is shown “Pretests passed”, but it doesn’t guarantee that your solution is fully correct. After Cf round there is system testing, in which your solution will be tested in the remaining test cases.