Hello,
A view state MAC is an encoded version of the hidden variable that a page's view state is persisted to when sent to the browser.
When you set the EnableViewStateMac attribute to true, the encoded and encrypted view state is checked to verify that it has not been tampered with on the client.
It is recommended to set the EnableViewStateMac attribute to true using the @ Page directive in the .aspx file.
I hope this information is enough for you.