Typically <spam> usually indicates a shameless self promotion or a tacky link to your website. This is against the forum's TOS.
There is a reputation system in place on this site. If people agree with your comment, they can reward you with points to improve your rep. If they dont agree, they can give you negative rep thus, you lose points. There is space to add additional comments if people agree/disagree with a certain part of your response. Not sure what the user who disagreed meant but maybe they thought you were trying to lead users to your site for the answer instead of giving it here. Again, not sure if that was why you received negative rep, just speculating.
Dont get too intimidated by rep nevermind negative rep. Keep posting and helping out and your positive rep will far outweigh your negative rep. Only link when there is a good reason to link and try not pointing that link to your site. This way, people wont think you have specific reason to post here [promoting your own site ]
"Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime." Chinese Proverb