What do you mean, lack of love needs to be filled?

The meaning of this sentence is as follows:

"Lack of love" refers to the imperfection and incompleteness in feelings, which may be caused by various reasons (such as emotional injury, divorce, breakup, etc. "Need to fill" refers to the need to take some actions or find the right person to fill these emotional gaps. This may include finding new love, repairing past wounds or enhancing one's emotional ability.