Refers to one's birth parent, as opposed to adoptive or foster parents. Often used in legal or formal contexts.
He doesn't know the face of his biological parents.
養子縁組では、実親の同意が必要な場合がある。
In adoption, the consent of the biological parents may be required.
実親 means one's biological parent(s), the opposite of 養父母, who are one's legally adoptive parents.
Compound of 実 (じつ, 'real, true') and 親 (おや, 'parent'). The reading じつ is the on'yomi of 実, while おや is the kun'yomi of 親, forming a standard jūbako-yomi compound.