To agree means to agree on something. Two or more people make a decision after a debate. They have reached a joint agreement and decide what to do. And that specific decision is what they have agreed on.
Human activities depend on regulations and norms that are the guidelines that govern an activity (a sports regulation, for example). The bodies that regulate the activities have to opt for those rules or conventions that are most appropriate for the purposes they intend to achieve.
When a professional group meets to share their knowledge, a convention is held; that usually includes workers of the same union or profession.
Convention is also used in one of the more classical debates in philosophy (nature versus convention, also called nature versus culture). The idea of the debate consists of defining what things are typical of the natural and which belong to the conventional, that is, to what is agreed by men. A concrete example of the controversy between the supporters of conventionalism and naturalism would be the valuation of the family. Conventionalists consider that it is a variable structure and each culture has its family model. The naturalists affirm the opposite: the family is is a form of universal organization and this evidences its natural character. This philosophical debate can be applied in other circumstances: on the laws, the conduct, etc.
People need to agree because they share information. Formerly there were many measurement systems (spans, elbows, steps ...). Each town used its system and it was not easy to change from one to another, necessitating some type of conversion. For this reason the meter was created, the conventional unit of measurement for most of humanity.
The new conventions are difficult to incorporate, because each group or sector defends its customs. This is what happens in Great Britain: there are those in favor of giving up their currency (the British pound) and using the currency of the European Union (the euro) and, on the contrary, there are those in favor of keeping the pound, a symbol of their national identity .
In social relationships there are unwritten rules about what is right and wrong; they are rules of coexistence. If there are a large number of social rules and customs and these are uncomfortable, we speak disparagingly of conventionalism, that is, an excess of obligations.
