Sunni are one of the two main branches of Islam, alongside Shia. Imagine that Islam is like a large family, and Sunnis and Shia are two different camps that have slightly different views on religious issues. Sunnis believe that after the death of the Prophet Muhammad, the best of the existing leaders should have become his successor, and that is how the first caliph, Abu Bakr, was chosen.
In the modern world, Sunnis make up about 85-90% of all Muslims, so their influence is quite significant. For example, many Muslim countries, such as Saudi Arabia or Egypt, are predominantly Sunni. This affects the politics, culture, and economic development of these countries.
If we look at something familiar, Sunni practice is somewhat similar to how there are different traditions in Christianity, such as Catholics or Protestants, who have slightly different ways of worshiping but believe in the same God. Thus, Sunnis, like all Muslims, follow the five pillars of Islam, which include prayer, fasting, pilgrimage to Mecca, charity, and the acknowledgment of the one God.
So, it can be said that Sunnis are the largest branch of Islam, which has a significant impact on how the Muslim world lives and develops today.