The continental effect is a phenomenon that causes temperatures in inland areas to fluctuate more than temperatures near to large bodies of water. Large bodies of water tend to have relatively stable temperatures while large bodies of land warm up and cool down dramatically. This means that the continental effect causes temperatures to vary more greatly than they would without that effect.
Large bodies of water like oceans can store a great deal of heat without really heating up. The oceans are vast and their waters churn around, mixing together and preventing any one area from getting much hotter than any other area. This means that the temperature of the ocean does not change greatly from season to season. Therefore, maritime cities tend to have temperatures that do not change as much as inland cities.
By contrast, large areas of land cannot really keep their temperatures stable. Instead, they get very hot in the summer and very cold in the winter. They have more variation in temperature between days (when the sun heats the earth) and nights (when it does not).