“April fish” is a joke that is made on the 1st of April in France. (“April fish” is a literal translation of the French “poisson” (fish) and “avril” (April).
On this day, everyone makes jokes - small lies are allowed, children and adults stick paper fish on the backs of the people they make jokes of. In the national media - newspapers, radio, television ... - are said fabricated “news”.
In France, this day has its symbol - the fish. That is why the jokes are called “April fish”.