The concept describing the systems of ideas, beliefs, and values through which people understand social and political reality — and the analytical framework for explaining why those systems tend to serve the interests of particular groups while appearing to be universal or natural.
Ideology is the concept that explains why intelligent people, looking at the same world, reach such different conclusions about it — and why those differences tend to correlate so strongly with where people sit in social hierarchies. Understanding ideology means understanding both how belief systems work and why they are so difficult to see clearly from inside them.