In software development agile is used to describe a set of adaptive methods to organise work. Agile is a way of thinking. A well known example of an agile method is Scrum.
According to the agile manifesto, a document written in the nineties by visionary software developers, there are 4 agile values. (1) Individuals and interactions over processes and tools, (2) Working software over comprehensive documentation, (3) Customer collaboration over contract negotiation, (4) Responding to change over following a plan.
Practically it means that a project is split into short, clear parts, that each result in a concrete result. Before a project starts nobody knows exactly what will be delivered in every part. This means the team can quickly respond to changing requirements and stakeholders can still steer the project during development.
The agile way of working really helped us to rapidly change the course of the project in the right direction.