Have you ever looked up a word in the dictionary and then had to look up three or four more words in order to understand the definition?  This is why these publishers have written learners’ dictionaries. 

• Macmillan • Cambridge • Oxford • Longman •

All of the definitions are carefully written using clear, simple language so they can be easily understood by people whose first language isn’t English.  Each dictionary has a unique style with their own advantages.  Visit their home pages and look at the different activities and exercises they have to get a feeling of which dictionary you like best.