They indicate replied, forwarded etc. I am hesitant to talk about their colours or even the direction they point in, as it can vary between versions of Thunderbird on different platforms and on what themes you are using. Many of them are described here: http://kb.mozillazine.org/Icons_-_Thunderbird_2 There is a new one, that comes with the recently added "Correspondents" column which tries to indicate whether it's an incoming or outgoing message.