Used to state clearly what the main difficulty or concern is in a situation.
“The issue is, our budget only covers half of what the caterer is asking.”
“The issue is that the tickets sell out within an hour of going on sale.”
Speakers often use 'the issue is' as a near-synonym for 'the problem is' to restate or narrow down the difficulty. If it's used a second time in the dialogue, it may be zeroing in on a more specific detail that the question is actually testing, so pay close attention to any change in wording.
Synonyms: the problem is, the difficulty is, what's tricky is