I know that salafis is a break off branch of the sunni branch, can anyone tell me how exactly different they are from us sunni? What makes them terrible? I'm a bit confused
Salafi isn't a break off from Ahlus Sunnah wa'l-Jamaa'ah (i.e., those who follow the Sunnah and stick to the Jamaa'ah).
"Salafi" refers to those who follow the Salafi Manhaj (methodology). The Salafi Manhaj = following the Qur'an and Sunnah according to the understanding of the Salaf (i.e., the first three generations of Muslims). This is because of the hadith of the Prophet ﷺ that states:
The Prophet (ﷺ) said: "The best people are those of my generation, and then those who will come after them (the next generation), and then those who will come after them (i.e. the next generation), and then after them, there will come people whose witness will precede their oaths, and whose oaths will precede their witness."
[Sahih Bukhari, 6429]
They had the best understanding of Islam as they were either contemporaries and students of the Prophet ﷺ or closest in time to him (and studied from his ﷺ companions or their students).
Salafis reject anything later introduced into the religion that does not go back to the first three generations mentioned by the Prophet ﷺ. The reason for rejecting later introduced matters being this hadith of the Prophet ﷺ:
It was narrated that Jabir bin 'Abdullah said: "In his Khutbah the Messenger of Allah (ﷺ) used to praise Allah as He deserves to be praised, then he would say: 'Whomsoever Allah (SWT) guides, none can lead him astray, and whomsoever Allah sends astray, none can guide. The truest of word is the Book of Allah and best of guidance is the guidance of Muhammad. The worst of things are those that are newly invented; every newly-invented thing is an innovation and every innovation is going astray, and every going astray is in the Fire.'
[Sunan An-Nasaa'i, 1578]
This is the main contention between them and Muslims who aren't upon this methodology and therefore follow practices that appeared later in Islamic history, e.g., Mawlid.