I heard from R’ Pesach Shafner shlit”a (this is my take on his words, please don’t blame him.)
The end of Maseches Chagigah brings a moshol:
אמר ריש לקיש: אין אור של גיהנם שולטת בפושעי ישראל, קל וחומר ממזבח הזהבֹ מה מזבח הזהב שאין עליו אלא כעובי דינר זהב, כמה שנים אין האור שולטת בו, פושעי ישראל שמלאין מצות כרמון, דכתיב (שיר השירים ד) כפלח הרמון רקתך, אל תקרי רקתך אלא רקנין שבך ־ על אחת כמה וכמה.
Reish Lakish said, “The fire of gehinnom cannot affect even the sinners of Israel, kal v’chomer from the golden altar. The golden altar had a gold coating no thicker than a dinar coin, and after many years the fire [on top] did not affect [the wood underneath]. The sinners of Israel who are as full of mitzvos as a pomegranate, as it is written..., how much more so!”
What is fire? You take some object that has a צורה, a form: say a wooden vessel. It seems to be completely stable and will last for years. Actually, it is not. Set it on fire, and you set off a chain reaction in the wood. The seemingly stable arrangement is changed into a more stable arrangement, a “lower-energy state”, with the extra energy released as more fire. That fire pushes more wood over the edge, until the entire wooden vessel is converted into a more permanent state: ashes and some heat, הבל.
What is the “fire of gehinnom”? That form we had in this world, the form we thought in our shortsightedness was so permanent - the fire of gehinnom dissolves it. Everything that is temporary is gone, nothing is left but the permanent. For some, unfortunately, that is nothing but ashes (Rosh Hashana 17a).
But the golden altar shows us that even a thin barrier of gold can protect from fire. If there is something stable, something permanent, the fire will not pass it. So too the sinners of Israel. Though they were very attached to the temporary, they are also full of mitzvos which are forever. The fire of gehinnom cannot harm them because they are really full of what cannot be burned.
There is a custom to light bonfires on Lag B’omer, the yahrzeit of R’ Shimon bar Yochai. I heard Rav Kostolitz shlit”a suggest a reason:
When R’ Shimon and his son came out of the cave, they saw people working their fields, instead of using all their time to study Torah. They said, מניחין חיי עולם ועוסקין בחיי שעה - “They are leaving behind the next world, the permanent world, and involving themselves in the world of the temporary!” כל מקום שנותנין עיניהן מיד נשרף - Everywhere they looked, burst into fire.
Later, when they came back out of the cave again, they saw a man involved in mitzvos - and they felt better. (Shabbos 33b)
[The gemara says they stayed in the cave for twelve more months because “The judgment of the wicked (Rashi, פושעים) in gehinnom is twelve months”!]