Sitecore Helix is a collection of recommended practices and conventions for the solution architecture of Sitecore product implementations.
These practices are based on principles of modular architecture (a.k.a. package design), which help you to manage dependencies in your solution in a way that creates more maintainble code.
Application of modular architecture will decrease the cost of change of your Sitecore solution, help you reduce technical debt, and provide more long term value to your customers.
These conventions also give the Sitecore Community a common terminology for solution architecture, making Sitecore implementations more discoverable for current and future developers.