Ethereum is an open-source platform, meaning that anyone can review, download, or propose changes to the underlying code that defines the rules and behaviour of the system. This is a common design of cryptocurrency protocols, and a precedent set by Bitcoin to foster transparency, collaboration, continuous improvement, and reduce the likelihood of bugs.
The open-source nature of Ethereum means that the codebase is publicly accessible and freely downloadable, allowing developers globally to contribute to its development. Anyone can audit the code to verify its security and rules. This level of transparency helps build trust within the community that the system will function as intended, given users and developers alike can see exactly how the protocol works.
Additionally, while Ethereum has had a roadmap of development since before it launched, anyone can propose changes to Ethereum through a process called Ethereum Improvement Proposals (EIPs). These proposals are reviewed by the community and the core developers, putting forth a distributed, community-driven approach to protocol changes. This process has led to major changes in Ethereum, such as the transition from Proof of Work to Proof of Stake and changes to ETH's supply policy.
By being open source, a community of developers is capable of building upon and auditing each other's work, without reliance on any one person, group, or company.