What means 7 green/blue loading bars in the top of the screen iMac 2021

Hi, community, I'm currently facing the next issue, sometimes the iMac starts normally, but most of cases it turn on but the screen is black, other cases it shows the apple logo, but at the top of the screen it shows 7 green loading bars, and after several restarts, it turn on normally, I don't think it's a screen problem due to when turn on normally the screen looks without any problem, but I'd like to know your opinion in case do you have a solution or facing the same issue than me, I already formatted my iMac and reinstall the SO, but the problem still happening.