"Please don't get angry," I throw out as a preemptive sally, even though it feels a bit like chucking an ice cube into the path of a forest fire.
What does it mean here?
... feels a bit like chucking an ice cube into the path of a forest fire.
The effort of extinguishing a forest fire with an ice cube will have no effectiveness. It might feel that the thrower has contributed something, but it is so minor as to be only symbolic.
This quotation is using a metaphor. (Specifically, it uses a simile. A simile is a metaphor that uses "like" or "as" to compare things.)
A forest fire "rages". It is huge, and dangerous. It can be "contained" by major interventions (such as back-burning, or dumping huge loads of water or foam).
A tiny fire (like a candle wick) can be put out by dropping an ice cube on it. Dropping an ice cube on a grease fire is likely to make it worse. Chucking (throwing) an ice cube at a forest fire is likely to make a tiny sizzle, but otherwise have no effect.
The author is saying that once the other person gets angry, their rage is very difficult to control. Saying "Please don't get angry" is very unlikely to prevent them from raging out of control.
The equivalent from Iceland is "Pissing on a lava flow."
Which is to say, to try to slow down a natural disaster with utterly inadequate means.
chucking an ice cube into the path of a forest fire
means "doing something that will make absolutely no difference to the situation"
A more common way of phrasing it is "throwing a snowball into the fires of hell"
An ice cube contains a very small amount of cold water. A forest fire contains a vast quantity of hot flame.
If you were to chuck (throw) an ice cube into a forest fire, it would have no effect on the forest fire. The ice cube would be completely destroyed in moments.