"Before Alamein we never had a victory. After Alamein we never had a defeat." What does this mean?
What Winston Churchill is saying here is that the Battle of El Alamein was a turning point in WWII.
This battle took place in October of 1942. Up to that point, Britain had had very little in the way of victories in the war. It is true that they had won the Battle of Britain and were no longer under threat of invasion, but that was about it. English cities were still being bombed. U-boats were still sinking shipping. Important British naval ships had been sunk by the Japanese who had also taken British outposts such as Singapore.
Therefore, it could have been said that the British had had no victories before El Alamein. After that battle, the war turned around. The British and their allies went on the offensive and gradually defeated Germany. They suffered no major defeats after this turning point.