NDAs bind one or both parties to keep certain shared information confidential. They're standard in customer-vendor relationships, M&A discussions, and design-partner agreements. They are notably uncommon in VC pitch conversations, most institutional VCs decline to sign NDAs at the deck stage because they see thousands of pitches and would create unmanageable conflicts.
NDAs are useful when you must share specific confidential operating data (customer lists, source code, financial details) and far less useful when sharing the high-level pitch story.