1. the most basic/important part of something upon which everything else is based
production was the cornerstone/bedrock of the business
A ‘cornerstone’ or ‘bedrock’ is something that acts as a foundation or base upon which other things are based or decided upon. Someone might be described as 'the bedrock of the community', for example - someone who organises events or who others depend on to get things done. If you say that something is 'the cornerstone', it means it is important and without it, other things would not be possible. This expression can be used in social and professional contexts.