Having a good knowledge of vocabulary and spelling is essential for reading and writing, and is an area which requires significant attention to improve spoken language too.
English is rich in wonderful vocabulary, with influences from around the world. It is considered a difficult language when it comes to spelling; words are often not spelt as they are pronounced, which is a challenge for people learning the language. However, this does not mean that you have to memorise a dictionary in order to learn. Learning word groups, prefixes and suffixes and word roots, and using phonics, can all make spelling and vocabulary easier.
And of course, context is important. You are more likely to remember words if you learn them in the context in which they matter to you.