Masarepa is precooked, ground corn flour that is used to prepare arepas. Arepas are a type of corn cake popular in both Venezuela and Colombia. Arepas were traditionally made by soaking the corn and then manually pounding the grains to remove the seed germ and the outer lining. The remaining part of the corn was then cooked and ground, and made into arepas. This labor intensive process is now done on an industrial level, so you can by the dried precooked corn flour and make arepas quickly and easily. Masarepa should not be confused wtih masa harina which is for making tortillas and is ground corn that has been treated with lime.
Masarepa can be found in Latin food specialty stores and online, and is also know as masa al instante and harina precocida. Common brands are Harina PAN, Areparina, Harina Juana, and Goya (which comes in both white and yellow varieties). Look for the words "harina de maiz refinada precocida," or "refined, precooked corn flour" on the package.