Quality post is opposite of spam post. I can explain about spam post. Just visit your post history, it's full by spam post. Now think how you can make quality post.
Merit is for quality and useful post. It doesn't mean all if your post will merited. If you made post in mega thread and your post is quality there is very low chance to get merit. Also you should post on appropriate board and post. Important thing is what you are contributing for this forum. There is any useful tips or topic for forum and forum members ? Useful and constructive post are consider quality post.