Na página
http://valgrind.org/docs/manual/quick-start.html#quick-start.intro
tem uma introdução de como ler essas coisas.
Nessa página ainda há uma sugestõe para que as mensagens indiquem melhor onde ocorreu o problema:
Compile your program with -g to include debugging information so that Memcheck's error messages include exact line numbers. Using -O0 is also a good idea, if you can tolerate the slowdown. With -O1 line numbers in error messages can be inaccurate, although generally speaking running Memcheck on code compiled at -O1 works fairly well, and the speed improvement compared to running -O0 is quite significant. Use of -O2 and above is not recommended as Memcheck occasionally reports uninitialised-value errors which don't really exist.