What is this smiley-with-beard expression: "<:]{%>"?
That's an empty lambda using a digraph disguise. Normal lambdas don't have beards.
The program uses digraphs to represent the following:
[] {};
This is a lambda expression that does nothing. The corresponding symbols have these equivalents:
<: = [
%> = }
Though they are generally unneeded today, digraphs are useful for when your keyboard lacks certain keys necessary to use C++'s basic source character set, namely the graphical ones. The combination of the characters that make up a digraph are processed as a single token. This in turn makes up for any insufficiently-equipped keyboards or other such hardware or software.
The program is using digraphs, which allow C++ programming with keyboards (or text encodings) that may not have the characters C++ typically uses.
The code resolves to this:
int main(){
[]{}; // smile!
}