An analogy. Think of a house. There is a way the house should be in
order to be a suitable shelter and living environment. If one wants
to make a house that is a good shelter (this is like wanting to make
oneself into someone who is in harmony with tian, someone who is ren),
then one needs to develop certain skills, such as plumbing, electrical
things, etc -- developing these skills allows one to know what to do
in order to build a good house and how to do it in the right way, in
the right order (e.g., need to set a foundation before building a
roof. Just as becoming a good carpenter is a way to build a good
house, developing li is a way to become ren.
Xiao is the virtue of filial piety -- the virtue of being loyal,
respectful, and obedient to one's parents. Xiao is another virtue
that one should develop in order to become ren -- knowing one's place
in the family, how to act as a good child, how to obey one's parents,
etc is helpful in knowing one's place in society, how to act as a good
citizen/friend/worker/etc and how to obey one's superiors.