Wittgenstein believed that all language is public, there's no meaning or understanding in the states or processes of the mind alone. Language cannot exist privately.

He emphasises this by considering it's opposite and ultimately denying it: the concept of private language

He explains it to be the following: it's the idea that language can be inherently private and known only to that individual. It's a theoretical language that refers to the person's private sensations, feelings or experience. And these references cannot be checked by anyone else. For example, I am experiencing a specific kind of pain that no one else has felt. I decide to call it ‘Flarg’. Only I know what ‘Flarg’ truly is because I'm the only one who's experienced it

Why does Wittgenstein deny this?

Let's take his analogy: let’s say everyone has a box with something called a beetle inside. No one can look in each others' box. My meaning of the word beetle is unique to me because only I can see what's in my box, and the same applies to everyone else. This, therefore makes the use of the word beetle inconsistent of it's use and meaning. We could all be looking at different things or the same thing but never know it.

He believes for language to be meaningful, there must be shared rules and criteria that others can use to verify what words mean. These words cannot be checked if they're in the private scope. Moreover, Following rules is a social activity, in which the community check and enforce if rules are followed.

So why has he made this distinction, what’s the point of emphasising that language is public?

Wittgenstein thinks the many problems we have in philosophy and other subjects comes down to how we understand language. It’s not merely a labelling mechanism of the world, nor is it a description of our behaviours. It is a way of participating in various forms of life and this participation is a communal activity.