What exactly are in-fills, how often do I need them and precisely why do I need them?

This is your maintenance cycle. If you always want a full set of dark, thick lashes, then In-fills are a must. They are used to fill in your lashes, during the natural lifecycle of your lashes. Every 2-4 weeks, you will go in and have in-fills applied to replace any lashes that have naturally fallen and where new lashes have grown.