Par value is the face value of a share that is set by a company when it first issues a new type of share. Par value may also be referred to as nominal value.
In most cases, shares are issued and allocated at a small insignificant value - for example at USD 0.01 per share. So in practice, a shareholder will pay very little for their shares, and won’t need to pay anything else in the future. Shares cannot be issued for a payment that is less than their par value.