The ahadith are the numerous collections recorded that narrate the sayings and actions of the Prophet Muhammad PBUH and his companions. Having ahadith is vital in Islam because everyone may have their own way of interpreting the Quran; many people will go to extremes and take verses out of context when interpreting the Quran. Therefore, the only way to properly interpret the Quran is to see how the Prophet Muhammad PBUH and his closest companions interpreted it. It should be noted that everything in the Quran is 100% authentic, but when it comes to the ahadith, some may be fabricated and made up by people. That is why there is something called hadith science, which is a grading system that classifies ahadith as authentic or not, based on chains of narrations and many more factors. Hadith science is an incredibly rigorous process, so much so that the vast majority of what humans know about history today would not pass the grading system in hadith science. This is because hadith science is based on authentic chains of narrations dating all the way back to the Prophet Muhammad PBUH, making it an astonishingly accurate system.
And Allah knows best.