What is the meaning of '[]' inside of input argument of the function in cpp

Question

First of all, I hope this is the right place to ask this kind of question. Pls, direct me if it is not.

This is probably the most basic question in c++. But, I have no clue what is the meaning of the line in the 'for' loop from the below code, and not sure how to search for it as well.

I guess 'for loop' iterates over the output of the vmap.findByFilter.

But what I do not understand is the input arguments([](const WhiteLine& white_line){return true;}) inside of it.

Can someone explain what does it do?

for (const auto& white_line : vmap.findByFilter([](const WhiteLine& white_line){return true;}))

Below is where the for loop is placed.

visualization_msgs::MarkerArray createWhiteLineMarkerArray(const VectorMap& vmap, Color white_color,
                                                       Color yellow_color){
  visualization_msgs::MarkerArray marker_array;
  int id = 0;
  for (const auto& white_line : vmap.findByFilter([](const WhiteLine& white_line){return true;}))
  {// some codes ~~ ~~~~    } 
}