Swizzling is a fancy word for changing the order of channels of a texture or a vector.
For example RGBA can be Swizzled to BRAG if you are feeling boastful. It's done for various different reasons like preparing a texture for compression or a certain function.
This is commonly done in a shader. More details can be found here: https://docs.microsoft.com/en-us/windows/desktop/direct3dhlsl/dx9-graphics-reference-asm-ps-registers-modifiers-source-register-swizzling