You need to understand first the dictionary meaning of word "liaison".
Liaison means communication or cooperation which facilitates a close working relationship between people or organizations. When you say that you are in to liaison or in to liaisoning, this means you are an intermediary. You are a mediator or middleman or representative, agent. You are a link.
Now the question on liaison in HR.
In HR, you deal with people. You deal with Human Resource. So you are a link between your department i.e. company and the Human Resource you have. You are an intermediary, you are a mediator, you are a middlemen or representative or agent.
In HR you also deal with various other agencies - statutory or non-statutory.
Statutory agencies are various government departments dealing with HR as well HR related laws commonly known as Labour and Industrial Laws.
Non-statutory agencies such as placement agents, HR outsource service and various vendors who provide you services in regard to health,safety, welfare etc. measures to your Human Resource.
Hope I answered your query.