As you can see moves are indicated if good or bad through symbols:

These are the symbols:

!!- brilliant move

!- good move

!?- interesting move 

?!- dubious move

?-bad move

??-blunder

Many people make blunders like the castling blunder like this one:

A brilliant move is performed by spectacular players like this one:

So as you can see think before moving one simple move effects the entire game