Prefixes are used in the API to denote specific semantic meaning of Vulkan names, or as a label to avoid name clashes, and are explained here:

Vulkan namespace All types, commands, enumerants and defines in this specification are prefixed with these two characters.
Function Pointer Denotes that a type is a function pointer, or that a variable is of a pointer type.
Pointer Variable is a pointer.
Commands that record commands in command buffers These API commands do not result in immediate processing on the device. Instead, they record the requested action in a command buffer for execution when the command buffer is submitted to a queue.
Structure Used to denote the VK_STRUCTURE_TYPE* member of each structure in sType