This mode tells the charger to only complete the first phase, constant current (CC) charging. The charger applies a high enough voltage to cause current to flow into the pack, up to the max current you've set. This gets the pack mostly charged, (70-80%) but the battery can still take more. Note, this fast charge mode has nothing to do with the charge rate, as that parameter can be set for any mode, and should be set based on your lipo, or at 1C if not specified. Fast charge merely tells the charger not to go on to the next (slower) charge phase, which brings us to:
This mode starts off like the fast charge, but after the cells reach 4.2V (or preset value) the charger switches into the constant voltage (CV) phase. In this phase the charger applies 4.2V to each cell and holds it there, while monitoring current flowing into the pack. This 20-30% capacity from the CV process can add quite a bit to the overall charge time, and actually increases if you used a high charge current in the first CC phase. Eventually the amount of current flowing into the battery drops lower and lower until the pack is considered "charged" and the charger stops completely. The LiPo can be considered fully charged at this point.
Finally, you may optionally choose to perform a balance charge if your charger supports it. This mode is similar to a normal charge, except that a small load is placed between each individual cell and is used to bleed off or equalize the voltages between these cells. This helps ensure one cell (with slightly higher voltage) isn't doing more/less work than the others, and thus dying faster than the rest of the cells. It is a good idea to perform a balance charge regularly if you want to extend the life of your lipo packs.
Thanks: Reddit User: Lord_swarley