As the solder connections get older, they will oxidize, crack and may lose their connection, thereby making your PC (desktop or laptop) either not work or not work correctly.
Reflowing a motherboard can be done in two different ways. One is by applying heat to each connection on the board and basically re-melting and cooling the connection. A soldering iron is used to do this. The person doing it has to be careful not to damage the attached electronic pieces or short any connections. The second way is to heat the whole board (with the meltable parts and burnable parts removed) in an oven, causing the solder spots to flex and reset with full connectivity.
Either way, I suggest having a professional do this. I have never done a reflow. Usually, when there are connectivity issues in the PC, I have found an easier step usually solves it, that is removing and reinstalling all cards and plug connections in the PC. These will oxidize also and can get dirt in them, also preventing connection.
Send your questions to Dwight Watt at email@example.com. He teaches at a technical college in northwest Georgia and does consulting work for businesses and individuals. His website is www.dwightwatt.com.