An error in the actual Java code. The code will not compile into an executable program, because there are errors in the code.
Compile Time Errors happen before the code even runs. The code is not able to run because there are errors in the actual text of the code, making it an invalid Java program.
Compile Time Errors make it so that your code cannot be compiled, which means it cannot be turned into an executable program.
Compile Time Errors include things like:
int x = "hello";)