Okay so my dear friends... I still can't read the 2nd season, I get the gist of what happe...

So, in season two it’s showing their past life, which is important context—it basically shows that the Kihoon from the past life found a rare blue glass rose to give to Yeon and Yeon told Kihoon to keep it for himself to remember him even when they were separated. Kihoon in the present remembers the past life because throughout his life he’s had dreams about it. So, the blue roses were to symbolize the glass rose from their past life and that’s basically the significance.