You can find all the compiler options with brief descriptions on MSDN:
C# Compiler Options Listed Alphabetically
Here are the descriptions for the ones in your question:
/optimize
Enables/disables optimizations.
/platform
Limits which platforms this code can run on:
x86, Itanium, x64, anycpu, or anycpu32bitpreferred. The default is anycpu./target
Specifies the format of the output file by using one of four options:
/target:appcontainerexe, /target:exe, /target:library, /target:module, /target:winexe, /target:winmdobj/unsafe
Allows unsafe code.