Everyone knows that ahlan wa sahlan أهلا وسهلا means ‘welcome’ in Arabic, but as with many phrases in the language, the more linguistically-minded student might ask what the story behind it is.
Ahl اهل means ‘people’ and sahl سهل means plateau or easy, and the reason for the mansoob منصوب construction is that the full phrase is originally:
حللت أهلاً ووطئت سهلاً
The original phrase is thus ‘[May you] arrive as part of the family, and tread easily [as you enter]’.
The phrase expresses a wish, and so is Optative in modality.