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
DELETE permissions denied on
User1 will not be able to delete data from that table.
Please see BOL's reference on Database-Level Roles:
public Database Role
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.