The top picture shows a series circuit. An electron that comes out of the battery has only one route to the other end of the battery. It never comes to a "fork in the road", and it never has to make a decision.
The bottom picture shows a parallel circuit. An electron that comes out of the battery immediately comes to a fork in the road, and it has to make a decision: "Should I turn now and go through the lower resistor, or should I keep going up for a while and go through the upper resistor instead". By the time the electrons return to the other end of the battery, some of them will take the lower route, and the others will take the upper route. The horizontal parts of their trips will be parallel.
