The lipid barrier is the protective oil-like layer that helps hair and scalp hold on to water. On the scalp, it supports comfort. On the strand, it helps reduce friction and keeps the cuticle from feeling rough.
When the lipid barrier is depleted, hair can feel dry even if you are using conditioner. The fix is usually not more product at random. It is gentler cleansing, less heat, and a routine that preserves the remaining barrier while replenishing what was lost.