In a nutshell, as I understand it:
What did the Meraglim do wrong, they were sent to give a report on the land and that’s exactly what they did?
The Ramban says that the problem was that the meraglim used the word אפס, which means impossible. The Akeidah says that they shouldn’t have editorialized the report with their opinion at all, they should have just reported the facts.
The Shelah says there were thee different groups:
Moshe wanted the Meraglim to report back on how strong the nations of the land were, so that the nation would greater appreciate the miracle that Hashem was going to perform for them.
The nation as a whole was in the middle – they wanted to make sure that conquering the land was naturally feasible, because they didn’t want to rely on a miracle. Their mistake was to take this too far, and to believe something that doesn’t seem possible is truly impossible, when nothing is impossible with Hashem’s help.
The Meraglim themselves thought they would lose their position of authority if the nation entered Eretz Yisroel, and they were therefore determined to convince the nation not to attempt this by whatever means necessary.