Affection is a stat that determines how much a girl likes you and their relationship level. There are currently a total of 8 relationship levels.
Once you completely fill the affection bar, the girl will reach the next relationship level. To get a girl to Girlfriend level and beyond, there are additional requirements that must be met. These requirements can be viewed by clicking on the girl's portrait in-game. They are also listed under each girl's page on this wiki.
Every action that recovers a girl's stats will raise her affection by some amount, as long as they have the energy and the stat is not at its max. Affection gain is reduced if the stats recovered by the action is greater than the max.
If you allow any of the girls' stats to reach zero, they will be forced to take care of themselves. While Pocket Waifu was under active development and events, ignoring your waifus and making them take care of themselves would cause a loss of affection. However, since the story and game has completed, there is no longer an affection loss if they have to care for themselves.
Another way to raise affection is by giving the girls gifts. Gifts always give full affection, even if the girl's Happiness stat is already maxed. If you have a lot of coins, buying and giving gifts is a good way to boost affection.