Further to the excellent post on the mechanics of flagging that @Nyx1819 covered above I would add a comment or two.
The standard by which flags are evaluated is the forum Code of Conduct (CoC).
Flagging a post on the basis that there is disagreement with what is said is not in itself a CoC violation and therefore, ordinarily, will not result in the flag being upheld.
For a post to get hidden then the moderator who handled the flag will have made a judgement call that the post in question is a violation of the CoC.
If one of your posts ever gets hidden as a result of a flag and you are unsure as to why it was hidden, then please send a PM to @moderators with a link to the post in question and ask for clarification on why it was hidden.
Often, if there is doubt as to why the post was hidden, the moderator will also have sent a PM explaining to the person the reason for the hiding of the post. In clear cases for a CoC breach a PM may not be sent beyond a system generated message. However there is still the opportunity to ask for further clarification, if required.
There is a responsibility on those who have flagging abilities to use that facility appropriately and abuse of the flagging mechanism does have consequences, as mentioned in the CoC.