#![cfg_attr(not(feature = "std"), no_std)]
Those are conditional attributes, a feature of rust. In a nutshell you say if
not(feature = "std") is true i.e. feature
std is disabled, then turn on attribute called
no_std module wide as indicated by the
! after pound
no_std prevents you from importing the rust std lib and making use of it. This is necessary for pallets as they run inside a wasm interpreter and do not have access to machine resources which is in turn required by rust std lib.
In short, it means: If
"std" is not enabled, flag/mark this crate/module as
no_std while compiling.
All you need is this: https://doc.rust-lang.org/reference/conditional-compilation.html