It means that make itself will ignore any error code from rm.
In a makefile, if any command fails then the make process itself discontinues processing. By prefixing your commands with -, you notify make that it should continue processing rules no matter the outcome of the command.
For example, the makefile rule:
clean:
rm *.o
rm *.a
will not remove the *.a files if rm *.o returns an error (if, for example, there aren’t any *.o files to delete). Using:
clean:
-rm *.o
-rm *.a
will fix that particular problem.
Aside: Although it’s probably not needed in your specific case (since the -f flag appears to prevent rm from returning an error when the file does not exist), it’s still good practice to mark the line explicitly in the makefile – rm may return other errors under certain circumstances and it makes your intent clear.