List.Select will analyze each item from the list.
"_" is the current item from the list.

What do you mean by current?

The second argument of List.Select starts an evaluation of the list, from the beginning of the list down to the last item in the list.

The _ is short hand for the current item during this process.

I suggest going more often to the documentation:
https://docs.microsoft.com/en-us/powerquery-m/m-spec-functions#simplified-declarations

The each-expression is a syntactic shorthand for declaring untyped functions taking a single formal parameter named _ (underscore).

Simplified declarations are commonly used to improve the readability of higher-order function invocation.

For example, the following pairs of declarations are semantically equivalent:

each _ + 1 (_) => _ + 1
each [A] (_) => _[A]
Table.SelectRows( aTable, each [Weight] > 12 )
Table.SelectRows( aTable, (_) => _[Weight] > 12 )

You should look here whenever you need something, all you need is documented:
https://docs.microsoft.com/en-us/powerquery-m/power-query-m-function-reference

Thank you very much for such a nice explanation.