The core principle behind agile software development is to promote flexibility and collaboration throughout the development process. Agile methodologies prioritize iterative progress, allowing teams to respond rapidly to changes and customer feedback. Key practices include breaking projects into smaller, manageable increments known as sprints, regular stand-up meetings, and frequent reassessment of priorities. This approach fosters continuous improvement and encourages active involvement from stakeholders, ultimately leading to higher-quality products and a more adaptive response to evolving requirements.