Learning vocabulary is one of the main activity of learning languages. The problem is that most people tend to focus only on vocabulary. They learn thousands of new words in a very short time, and after a few days they forget everything they have learned.
It’s good to learn vocabulary, but the most important is to use it in context. Our brains remember much more efficiently words that are useful for us. If you learn a word, but you never use it, you will forget it immediately.
There are many techniques to memorize words on the long term. The most efficient is probably spaced repetition. There are many software like Anki or Mosa Lingua that use spaced repetition to learn.