What a hard question!
Operation Overlord was the time when the British, Canadians and Americans invaded Hitler's 'fortress Europe'.
They invaded so that they could win the second world war.
Britain's involvement was a result of her treaty obligations to Russia and America - so you will need to know about things like the Atlantic Charter and the Casablanca Declaration.
At the time, Stalin was invading from Russia - and defeating Hitler. He had been asking for a 'second front' since 1942, instead the British and Americans had begun Operation Torch in North Africa; the Allies were pushig up through Italy, but making slow progress. D-Day marked the real opening of a second front.
You will find some of these ideas - in a muddled sort of way - on
this webpage.
There was an element of fear in the Allies' action - Stalin was defeaing Hitler. They invaded to win WWII, yes, but they also invaded to get as far across Europe as possible before Germany collapsed, and thus to stop as much of Europe as possible falling into Staln's hands; THAT was why they rushed too fast, and that explains the problems at Arnhem, which was 'a bridge too far'. This idea is explored in
this webpage.
This cheat essay approaches the question in a very narrative way. But you can use it to steal some facts to support your ideas.