Herbert in his book "Elements of set theory" on page no 3 says that
we can form the set $ \{ \emptyset \} $ whose only member is $\emptyset $. Note that $ \{ \emptyset \} \neq \emptyset $, because $ \emptyset \in \{ \emptyset \} $ but $\underline{ \emptyset \notin \emptyset} $·
By the last argument $\emptyset \notin \emptyset$, is he saying that empty set is not a member of or does not belong to empty set
OR
it is a typo and he wanted it to be $ \{\emptyset \} \notin \emptyset $, that set containing empty set is not a member of empty set