What is the significance of the site of Harappa in relation to the Harappan culture?

It is the first site where a unique culture was discovered.

The Harappan culture, also known as the Indus Valley civilization, is identified by archaeologists as a distinct cultural group. The term "culture" refers to a collection of objects that share a distinctive style and are typically found together within a specific geographical area and time period. These objects include seals, beads, weights, stone blades, and even baked bricks. They have been discovered in various regions, spanning from Afghanistan, Jammu, Baluchistan (Pakistan), to Gujarat

The name "Harappan" derives from Harappa, the initial site where this unique culture was first uncovered. The civilization is estimated to have thrived from approximately 2600 BCE to 1900 BCE. Within the same region, there were earlier and later cultures referred to as Early Harappan and Late Harappan. The term "Mature Harappan culture" is sometimes used to distinguish the Harappan civilization from these earlier and later cultures.