This, sometimes expressed as “you have an agenda”, is an example of the ad-hominem fallacy. It is often used by people as a failed attempt attempt to discredit an argument. When it comes to determining whether something is true, it is mostly irrelevant who said it. What matters is whether they provided evidence to back up their claim, and whether they are open about how that evidence was collected.
The best way to refute a claim is to produce counter-evidence to show that the claim is incorrect. When somebody resorts to the ad-hominem fallacy, it suggests that they have no basis or evidence to support the point of view they are putting forward, so can probably be ignored.