Grant tables contains the grant information of

user accounts,global privileges and other non-privileges
database level privileges;
table level prtivileges;
column level privileges
proxy user privileges

Indirect modification of grant tables can be done by using Create User,Grant,and revoke.

Direct modification of grant tables can be done by using update,Insert and Delete.

Skipping grant tables disables the password login for the root account and also restarts the server where with the help of this we could reset the password for the root account.