Meaning
A simple yellow face with a small closed-mouth smile and plain dot eyes. On paper it means mild contentment or polite friendliness, the visual version of a soft “sure” or “thanks.”
In practice its meaning depends heavily on who uses it. Many older or more formal writers send it sincerely, as in “See you at 9 tomorrow 🙂” Many younger users, though, read the frozen smile as passive-aggressive, the face you make while hiding irritation. If there is any tension in the conversation, a warmer face or a few extra words avoids that misreading.