In the title 『四月は君の嘘』, the 「は」 is the so-called "topic marker" in Japanese. It is a particle of speech that introduces the "topic of discourse", and is a fundamental element of Japanese grammar. In this sentence, it basically tells the listener that the context of "Your Lie" was "April".
「は」 is probably one of the first things you encounter when studying Japanese, so I am guessing you are relatively new to the language. It is – unfortunately! – not necessarily easy or quick to understand/explain to English speakers, as there is no direct equivalent in English. Nevertheless, to aid beginners with understanding how it is used, resources often suggest you structurally translate 「は」 to "As for" in English. So, structurally, it might help you to understand the sentence as "As for April, Your Lie".
I understand this is rather fuzzy in meaning, but this does also reflect the fact that the phrase in Japanese would also change its meaning depending on the context; for instance, it could be translated to any of "Your Lie in April", "April is Your Lie", or "Your Lie was in April", amongst others.
Of course, "As for April, Your Lie" sounds completely unnatural in English. "Your Lie in April" is a much more natural and stylish translation. One would expect this translation decision to reflect the creators' intended meaning of 『四月は君の嘘』 in Japanese, but it's impossible to know for sure!
There may be a more nuanced explanation/interpretation as to why the Japanese creators used 「は」 over other possible choices in Japanese, but I don't think that's strictly in the scope of your question, so I shan't digress! Hope this helps.