From the GitHub documentation:
Development containers, or dev containers, are Docker containers that are specifically configured to provide a fully featured development environment. Whenever you work in a codespace, you are using a dev container on a virtual machine.
Currently the template, and packages created from it; are using the default devcontainer when a GitHub codespace is opened; and thus install the project dependencies and VSCode setup when they start using that space. To define the container in which to develop a project, and also to be able to do that locally, we can defined a devcontainer configuration. This can then be utilised to configured codespace initialisation and for local development - saving valuable time that could be spent developing and also reducing environment-related issues from developing in non-configured, non-isolated local environments.
From the GitHub documentation:
Currently the template, and packages created from it; are using the default devcontainer when a GitHub codespace is opened; and thus install the project dependencies and VSCode setup when they start using that space. To define the container in which to develop a project, and also to be able to do that locally, we can defined a devcontainer configuration. This can then be utilised to configured codespace initialisation and for local development - saving valuable time that could be spent developing and also reducing environment-related issues from developing in non-configured, non-isolated local environments.