thing is

Từ điển Oxford Advanced Learner 8th

▪ I. the ˌthing (about/with sth/sb) ˈis idiom
used to introduce a problem about sth/sb
• The thing with Karl is, he's always late.
Main entry: ↑thing_idiom
 
▪ II. the ˌthing ˈis idiom
(informal)used to introduce an important fact, reason or explanation
• I'm sorry my assignment isn't finished. The thing is, I've had a lot of other work this week.
Main entry: ↑thing_idiom