used to say that sth is possibly true but not definitely or always true Examples: The more expensive articles are not necessarily better. Biggest doesn't necessarily mean best. ‘We're going to lose.’ ‘Not necessarily.’ Source: Oxford dictionary

It means "I don't agree with this in all possible cases." or "I might agree with this only if X happens or Y changes." As Nico pointed out, sometimes native speakers will use the word "necessarily" in a flippant manner, but that isn't always (or even often) the case. You won't always necessarily know whether the speaker really meant 'necessarily' or not. :)

Hi It simply means the speaker does not agree with something. The "necessarily" is not needed, but commonly used.