Paul is explaining that an idol, of and by itself, is nothing at all, except that it is made to seem important by peoples' imaginations. Paul explains that some people don't understand that this means that meat sacrificed to idols is the same as meat that has not been sacrificed to idols. Because it might encourage the less knowledgable Christian to sin by eating the meat before they are fully convinced in their own heart that it is ok. Thus it would cause the weaker brother to sin. This not only illustrates that a Christian can and does sin, but it also illustrates quite well what sin is, and also what it is not.
Sin occurs when the conscience is violated.
The act itself is secondary to the individual's perception of the act. This is not easily accepted by the person who has approached Christianity in a legalistic manner. In fact, we should always be aware of the context of every scripture that is used to teach truth. Everyone should prove for themselves whether these things are so, or whether the scriptures are being misapplied and misused. And lest, when I come again my God will humble me among you, and that I should mourn over many of those who have sinned before and have not repented concerning the uncleanness and fornication and licentiousness that they have practiced.