Wartime is the period when a country is fighting a war. It is uncountable, and it usually appears without "a" or "the": "in wartime", "during wartime", "food was short in wartime".

Much more often, though, you will meet it in front of another noun, describing something that belonged to that period: wartime Britain, wartime rationing, wartime memories, his wartime experiences, a wartime romance. When it works this way it means "from the war years" or "happening during the war", and it quietly carries the whole atmosphere with it — shortages, danger, families apart. That is why writers like it. "Her wartime childhood" says in two words what would otherwise take a paragraph.

Its opposite is peacetime, and the two are often set against each other: "a factory that made cars in peacetime and planes in wartime".

One thing worth noticing: in English-speaking countries, if someone says "the wartime years" with no other information, listeners usually assume World War II. It is not part of the dictionary meaning, but it is how the word lands.