What does -했을 거예요 mean?

Hello! Recently, I have become curious about various expressions while learning Korean. I heard the sentence "오늘 많이 미숙했을 거예요" while watching a cooking video, and this expression confuses me a bit. Normally, -을 거예요 indicates the future and -했은 indicates the past, but when these two are combined in -했을 거예요, I would like to know what meaning it has. From what I understand, -했을 거예요 seems to express speculation or possibility about the past, but I am not sure about the exact usage or nuances. I am curious about how this expression is used in everyday conversations or specific situations in Korea. If anyone knows the exact meaning and usage of this expression, I would appreciate an explanation. Especially if you could explain it in an easy way for foreigners who are not familiar with Korean, I would be grateful!