what does this line means #![cfg_attr(not(feature = "std"), no_std)]?

Question

#![cfg_attr(not(feature = "std"), no_std)]

Answer 1

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 #. Enabling 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.

Answer 2

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