What are the "monument" races?
In the world of professional cycling, the 'Monument' races are five of the oldest and widely considered most prestigious one-day races held each year. They are:


Milan – San Remo (Italy) 
Tour of Flanders (Belgium) 
Paris–Roubaix (France) 
Liège–Bastogne–Liège (Belgium) 
Giro di Lombardia (Italy)


More information can be found on each race, and the "Classics" in general in the Wikipedia article Classic Cycle Races. 

A key factor in the argument that Eddy Merckx is the greatest competitive cyclist ever is that he won 19 Monuments - the next rider on the list won 11.