Question

Pls explain it from Valency is ............. a standard.

Answer 1

See,
Valency is present in the outer most orbitals
For eg: valancy of oxygen is 2, that means 2 more electrons is needed to complete the octect.

Lets say valency of Li is 1,
Atomic number of Li is 3, by losing 1 electron,it will attain noble gas configuration

In your text , there explained old definition of valency

Answer 2

Nothing is difficult in that bro!! What they wanted to tell is that valency of an element is measured in terms of hydrogen atoms or oxygen atoms. It also says that valency of an element is measured in terms of hydrogen atoms or double the number of oxygen atoms that can combine with the element.
Hydrogen has the least valency of 1 because it can combine with only one more hydrogen atom to give hydrogen molecule. The Valency of Hydrogen as 1 is considered to be standard.
Hope it helps you