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 The purpose of the junction box except converting the ribbon into cable is to protect the module and make it more effective, the main component here is the diode, see picture. To explain, first one have to understand the structure of the module and how the cells are connected to each other in the module.
The cells in a typical module are serial connected and follow the pattern seen in figure 1.
 With the serial connection of cells it is possible to make a connection point each time the serial connection come back to the side of the module in which the serial connection begins. As seen in figure 2, this creates three sections of the module. With the junction box connected, each section becomes parallel to a diode, see figure 3.
In normal operation mode, current will flow through the module without going through the diodes. However, if let’s say a bird dropping cover one of the cells on the module, the sunlight will not be able to penetrate that cell, and the cell will stop produce current. At this point the cell stop working as a producer of energy and start to work as a resistor. Two bad things comes with that, the first thing is that that cell will become very hot, in some cases so hot it will break down, the second issue is that the efficiency of the module will decrease a great deal in order to push the current through the disabled cell. With the diode in parallel connection to the section that hosts the disabled cell, the current will flow through the diode instead, hence the word bypass diode. See figure 4.
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