It increases each individual Poison stack by +1.
Each Noxious Fumes turn is +1 poison to all enemies. Playing multiple Noxious Fumes will still only grant +1 poison. So a Noxious Fumes of 4 would only deal +5 poison per turn.
Each Bouncing Flask attack is +1 poison (+3 in total, or +4 in total when upgraded.)
Each Envenom strike is +1 poison (granting +2 poison each in total, effectively doubling Envenom.)
When playing Catalyst, +1 poison is added AFTER the current poison is doubled/tripled. For example, if your enemy has 8 poison currently, and you play an upgraded catalyst, they'll have 25 poison.8*3+1.