'simpleSecurityObject' requires attribute 'userPassword'

[Villem Alari]

Entry (cn=admin,dc=example,dc=com): object class  
'simpleSecurityObject' requires attribute 'userPassword'
slapadd: dn="cn=admin,dc=example,dc=com" (line=12): (65) object class  
'simpleSecurityObject' requires attribute 'userPassword'

Why does it say that 'simpleSecurityObject' requires attribute  
'userPassword'. What password it means? I have done everything  
exactly as it is said here till the point where it says add the new  
content sudo slapadd -l init.ldif. After doing that this  
'userPassword' appeared.

Regards,
Villem Alari