This means that if the user has not had permissions explicitly set for the securable (or inherited from permissions explicitly set) that **public** has permissions for, then those will be applied.  In other words, if you have `User1` with `DELETE` permissions denied on `User_Table`, then `User1` will not be able to delete data from that table.

Please see [BOL's reference on Database-Level Roles][1]:

***public* Database Role**<br/> 
*Every database user belongs to the public database role*. When a user has not been granted or denied specific permissions on a securable object, the user inherits the permissions granted to public on that object.