As you may already know, amino acids are the building blocks of all proteins.
There are 20 amino acids in total, 9 of which are essential, meaning that the body needs them, but can't produce them on its own.
And while animal products like meat, organs, eggs and dairy, contain all essential amino acids, the same doesn't really go for plant foods.
Most plant foods lack certain amino acids, which makes them incomplete, when it comes to their protein content.
For this reason, plant protein sources are classified as being of a lesser quality and biological value for the human body, as opposed to animal products.
Unfortunately for some, this will never change, but if you are a plant-based eater, your best bet is to combine different plants, in order to make them compensate for one another's lacks.