When speaking of an object, a person or a situation as 'conventional', reference is being made to the notion of convention, that what is being talked about follows the established conventions for such a case and therefore cannot be considered as different or alternative. The notion of conventional is especially complex when applied to issues and situations related to society and its organization since conventions can be consciously or unconsciously established by it and its respect or not is what makes an individual fall outside of socially established norms.
The idea of convention has to do directly with that of agreement. This is so since a convention can never be the opinion or decision of a single person, but rather the agreement that occurs between several people at the same time. Social conventions always imply forms of behavior, attitudes, values and norms to be followed that return to conventional situations, people or objects according to whether they follow those rules or not.
Normally, the conventional elements of a society are those that nobody disputes and that can always be present, tacitly or explicitly. However, what is conventional for one society (for example, dressing in a certain way in a certain situation), may not be conventional for another community and therefore coexistence can become conflictive in certain cases. This type of conflict can also exist within the same community between the different social groups that form it, between different age groups, between men and women, etc.
Finally, we can add that social conventions and the conventional is what allows a society to function in a more or less organized way in a certain way. Respecting certain conventions and complying with the rules of behavior entail acting for the common good of all the members that are part of society or of a certain institution (such is the case of members of the army). Breaking with these rules of conventionality always implies a significant spirit of rebellion because generally most people choose to follow these conventions with little discussion.