1. What does it mean?
레알 is a playful form associated with English 'real.' In informal Korean it can reinforce a statement or ask whether surprising information is actually true.
Also written or said as 리얼, real, 레알이야
2. The nuance
A standalone 레알? is a quick 'for real?' rather than a request for a detailed explanation. In a sentence it often adds emphasis. Football contexts can instead be referring to a club name such as Real Madrid; those are not the slang sense.
3. Examples
“그 긴 보고서를 혼자 끝냈다고? 레알?”
You finished that long report by yourself? For real?
“이 의자는 오래 앉아 있어도 레알 편하다.”
This chair is seriously comfortable even when I sit for ages.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.