Civil-military relations mean the relationship between people who have the power to make decisions in the government (called civilians) and people who have the power to protect the people and the country through the use of force (called military).
It is important because both groups work together to keep the country safe and the government functioning properly. However, there need to be rules and procedures in place to make sure that everyone is doing their job correctly and that the military doesn't take over the government (which is a big no-no).
In short, civil-military relations is like a friendship where both parties have to understand and respect each other's roles and responsibilities to achieve a common goal.