What does the plus (+) mean in a voltage source symbol?
When a voltage source is said to be 5V, it means that the plus side will be 5V higher potential than the minus. If it was -5V, then the plus side will be 5V lower. Remember, voltage is always relative. The main convention is minus is always the reference. 

It's good to stick to one perspective. Most things are based on conventional current, the flow of 'positive charge'. It is 'positive charge' that flows from higher to lower potential. A system based on electron current will invert everything.