Some context first.
OpenGL is an API to program graphics cards. The API calls to OpenGL get translated into commands for the GPU card in the computer, by the driver of the GPU vendor (usually Nvidia, AMD or Intel).
(other APIs to program GPUs are Direct3D on Windows, Metal on macOS, Vulkan, etc.)
These APIs are somewhat "low-level", in the sense that they give calls to instruct the GPU what to do at a raw "geometry"/"pixels" level; these APIs don't have anything like "draw a rectangle here", or "draw this image there".
Web browsers like Chrome provide the WebGL API for HTML5 pages. WebGL is mostly the OpenGL ES 2.0 API.
ANGLE is a library that converts the OpenGL ES 2.0 API to one of the GPU APIs: you make OpenGL ES 2 calls and ANGLE converts to equivalent calls in desktop OpenGL, or to Direct3D on Windows, or to Metal on macOS.
This gives you a consistent implementation of OpenGL ES 2.0, which Chrome wants for the web. (This is super useful because the Windows OpenGL implementations differ from vendor to vendor, and macOS stopped updating its OpenGL implementation at version 4.1 a long time ago).
Skia is a higher level API that lets you "draw a rectangle here" and "draw this image there" and much more (draw text; draw paths; draw curves; etc.) This is the usual high-level API that you'd get in higher level languages, and on the Web Canvas API.
To implement those calls, Skia converts the drawing commands to lower-level APIs like GL, Metal, etc. One of the APIs that Skia can target is ANGLE, so that Skia drawings get consistent output across platforms.
