Upright meaning
Ice is a broad bridge and a treacherous floor. Things are frozen for now. Stillness can carry you across, but test each step.
Reversed (merkstave)
Íss looks the same either way up, so it has no reversed meaning. Read it upright every time.
A question to sit with
Is this stillness a rest you need, or a thaw you are avoiding?
Its ætt
Íss belongs to Hagall's ætt, the runes of forces outside your control and the growth they force.