The javac flags are codes for specifying compilation options. For example, if you type...
javac -verbose MyFile.java
Then the
-verbose flag tells the compiler to output messages while compiling (e.g., what classes it's loading).
A basic list of options is displayed by typing javac -help...
For more detail, see
javac - Java programming language compiler.
(Note that there are also flags for Java runtime. For example, the
-ea flag enables assertions. Type
java -help for a basic list.)