Read the full text of Cymbeline Act 1 Scene 6 with a side-by-side translation HERE.
The Queen shoos her ladies away so she can talk one-on-one with Dr. Cornelius. She's asked him to make a secret potion that will kill anyone who drinks it. Dun-dun-dun. She just wants it for scientific purposes, of course.
Cornelius is no fool. He doesn't trust the Queen to use the poison on animals like she claims. So, he's actually given her something that will make it look like someone is dead. Then, if the Queen gives it to someone, that person can be revived from a deep sleep.
After Cornelius tells us this secret, he leaves.
It turns out Cornelius's instincts were right, because just after he leaves, the Queen gives the potion to Pisanio. She tells him that it's an energy drink. After all, it's saved Cymbeline's life five times already.
Why does the Queen want Pisanio dead? That's simple: he's always praising Posthumus to Imogen. He's all, "Posthumus can do this, and Posthumus can do that. Look how amazing Posthumus is."
The Queen figures that if Pisanio's out of the picture, Imogen will forget all about Posthumus. Then Imogen might actually give the Queen's son Cloten a chance. Logic does not appear to be the Queen's strong point.
As if giving Pisanio a poisonous drink weren't enough, the Queen also asks him to talk Cloten up to Imogen.
After the Queen leaves, Pisanio's like, "As if." He'll never betray Posthumus and help the Queen.