This is kind of obvious, but I cannot imagine what I would have done without my dictionary on my phone. Wherever I am, I can check any word I see or hear, it’s really one the most helpful tools you can have. My dictionary’s called JED, on Android.
I think it’d be a good thing to have a dictionary on which you can search a word either in English or in Japanese with any alphabet you want and where you can search Kanji you don’t know with their radicals. The latter is extremely helpful when you find yourself before a word with a Kanji you don’t know. You can find said Kanji by selecting radicals on the app and then know the readings so that you can search the word in the dictionary and finally know what it means. JED does all that, which is why I’m using it, I highly recommend it.
Obviously there are words that are hard to translate and some have different meanings in different contexts, but even though, looking up a word or a grammar structure in the dictionary always helps to get an idea of the meaning and clear things up.
Another good thing about having a dictionary is that once you download it, you don’t need an internet connexion anymore. If you were to use an online service like Google Translate for instance, if you find yourself in the middle of nowhere, or just somewhere where there’s no Internet connexion (my school for instance, that’s right), well you’re happy you have your dictionary, you can check a word or Kanji at any time. Even during class when you’re phone’s offline so it doesn’t ring. You can still check stuff that you don’t understand.
Plus, compared to a paper dictionary (which costs money, the app is free), it’s much faster to check a word because you can directly type it in, rather than turning pages for sometimes a long while.
In my opinion, a dictionary on your phone is the must have app for when you’re learning a foreign language.