Could mean any number of things in context. In this case it seems to be DPI aware, have some guarantees on memory safety, uses Vulkan/Metal, and targets the current versions of the supported systems natively.

This would be opposed to a legacy codebase that might rely on an old version of OpenGL (which is deprecated completely on MacOS), the OS to scale the window (usually blurring the pixel art), require 32 bit architecture support, and not have any guarantees about memory safety.

As time goes on what features are considered modern will of course continue to change. Most code was modern at one point.