Q: The meraglim complained and that's why we have Tisha B'av. What does it mean to complain?
A: The story of the meraglim has a lot of depth and there are loads of meforshim, explanations, on every passuk. It's not just that they complained. The spies were sent out on a mission to look at Eretz Yisroel and come back with a report.
Hashem did so many things along the way to make it positive but all they noticed was the negative. He caused many people to die so the inhabitants would be busy taking care of the dead and not notice the spies; instead they saw it as a land where people are always dying. (In todays terms, you can look at it as a land full of epidemic and not a healthy place to live.)
Hashem made it take shorter to travel across the entire land and let them see the most beautiful and fruits - they were huge. They took that to mean that this is a land full of giants and strong people; it would be too difficult for us to conquer such a land.
They didn't look at Hashem's bracha with a positive eye and didn't trust that He would help them fight to get in. So when they complained and cried-about something that was in essence a huge bracha, Hashem said He will make that night into a night where our tears will flow.
There is for sure a lot more to say but this is just the tip.
It's not just the complaining that brought about Tisha B'av. It's that they cried on that night, so it became a night where bad things happen to us.
It's the whole story of the spies going out with a negative attitude, coming back with a report that made the entire nation terrified to go into the land Hashem had promised us all these years! The entire nation cried and weeped and sobbed all night-from fear of having to enter this special land.
The spies took away everyone's trust that when Hashem promises something it's always good. The people didn't want to go into Eretz Yisroel! They didn't want Hashem to fulfill His word - all because the meraglim came back and said so much negative about Eretz Yisroel, Hashem's chosen land, for His chosen people.
The spies said bad things about the land-but in reality all those things were really good. They made Bnei Yisroel lose Hashem's trust by saying things that weren't 100% true in the first place. Everyone cried over something they should never have been crying about!
This is what it means that they cried for nothing.