Software development is better if it's fast, because it's all temporary anyway.
Agile software development is an often disputed concept that means many different things in practice, but broadly, it is an iterative and incremental approach to building software. It was developed as a reaction to the planning heavy "waterfall" approach that many people believe is not well suited to the inherent, and unpredicitable, complexity of building large software systems.
In waterfall development, you start with planning and end with a product.
In agile development, you start with planning, end with
version 0 of the product, get feedback on it, build
version 1, get feedback and on and on until people are happy with it or you run out of money or time.
Agile is generally regarded as a good set of principles, but no number of agile coaches can fix a bad business model.