The phrase "inherent bias" refers to the effect of underlying factors or assumptions that skew viewpoints a subject under discussion. There are multiple formal definitions of "inherent bias" which depend on the particular field of study.
In statistics, the phrase is used in relation to an inability to measure accurately and directly what one would wish to measure, meaning that indirect measurements are used which might be subject to unknown distortions.
References
Inherent bias Wikipedia(Text) CC BY-SA