Volatility is another word for variance.
The term variance is often used in video poker circles but it applies also to slots.
Basically, volatility means just that. You will experience some big swings in your balance when playing.
I think what people understand the least is the relation between volatility and RTP.
People who play at 3Dice would know that the higher volatility slots have a higher RTP.
There's a reason for this; it takes more spins to reach the maximum prize.
Let's take for example a low volatility slot with an RTP of 90%.
On this slot it takes on average 10 thousand spins to hit to top prize.
So you can expect the slot to reach it's RTP of 90% at 10 thousand spins.
If you were to play this slot at $1 a spin and stopped once you got the top prize you can expect to lose one thousand dollars (10% of 10 thousand.)
A medium volatility slot might take 100 thousand spins to hit the top prize.
In this case the slot has an RTP of 99%.
If you were to keep playing at $1 per spin and stop once you get the top prize.
You would lose one thousand dollars (1% of 100 thousand.)
90% or 99% you are still losing a thousand dollars.
And that's why we see the more volatile slots given higher RTPs.
Even though the RTP may be higher on some slots the casino is still making the exact same profit and the player is still losing the same amount.
I think slot designers are taking advantage of this gap in player knowledge.
One slot designer comes to mind: Big Time Gaming.
In my opinion, they have the volatility set so high on their slots that it could justify a 99% return.