"Protein filler for hair" falls under the part of speech _noun_, referring to a substance applied to hair to replenish lost proteins. One common example is a keratin treatment, which uses a protein naturally found in hair to smooth and strengthen it.
Protein fillers play a significant role in hair care, as hair is primarily composed of proteins. When hair is exposed to heat, chemicals, or environmental factors, these proteins can become damaged and depleted. Protein fillers help repair and restore damaged hair by replenishing lost proteins, resulting in improved hair health, strength, and appearance.