A conceptual model is a graphical overview of the aspects of a research and how they are related to each other.
In any research a lot of aspects are described and the interrelations between the aspects are being investigated. Very often this is done in an exploratory or in a descriptive research. These types of research have a design with only one observation moment, but even in a single moment a lot of aspects can be part of the study. To give an overview of all relevant aspects a conceptual model is created. The conceptual model is usually based on a literature study, but it does not have to contain all aspects of a theory. Besides that, even new aspects may be added. Read more about these topics on the pages Theory and Model.
Read more about the longitudinal design in our paper: How to create research designs and conceptual models.