Strange expression found in std.variant

[Francis Nixon via Digitalmars-d-learn]

When looking at std.variant I found the following line:
return q{
     static if (allowed!%1$s && T.allowed!%1$s)
          if (convertsTo!%1$s && other.convertsTo!%1$s)
              return VariantN(get!%1$s %2$s other.get!%1$s);
}.format(tp, op);

I was wondering what exactly the % signs where doing/what they 
are for?