This is the 5578th most frequent Arabic word.
فحسب
"Only" or "just."
فحسب here indicates limitation, showing that the speaker only desires one thing, a sandwich, and nothing more.
أريد شطيرة فحسب.
I just want a sandwich.
فحسب here emphasizes the exclusivity of the action, indicating that the person was limited to smiling and not doing anything else.
كان يبتسم فحسب دون أن يتحدث.
He was just smiling without speaking.
فحسب in this sentence restricts the scope of the problem to misunderstanding only, excluding other potential issues.