When someone says, "To say I miss you is an understatement," they're expressing that their feelings of longing and affection for someone are so strong that the simple phrase "I miss you" doesn't capture the depth of their emotions. It's like saying, "I miss you immensely." This quote is used to emphasize just how much someone yearns for the presence of the person they're talking to.
Imagine you haven't seen a close friend or loved one for a very long time, and you've been thinking about them constantly. Saying, "I miss you" might feel like an insufficient way to convey the magnitude of your feelings. So, you use the phrase in the quote to show that your longing is beyond what mere words can express.
In essence, "To say I miss you is an understatement" emphasizes a profound sense of longing and affection, indicating that the speaker's emotions go far beyond the simple words they're using to convey their feelings. It's a way of expressing deep affection and a strong desire to be with someone you care about deeply.