If an effect would set your life total to a specific number that's higher than your current life total, that effect causes you to gain life equal to the difference. If your life total is 5 or less, Phial of Galadriel will then double the amount of life that effect would cause you to gain. For example, if you have 3 life and an effect says that your life total "becomes 10," you would gain 7 life which Phial doubles to 14, so your life total will actually become 17.