A lexicon is often used to describe the knowledge that a speaker has about the words of a language. This includes meanings, use, form, and relationships with other words. A lexicon can thus be thought of as a mental dictionary.
A successful learner develops an L2 lexicon independently of their L1 lexicon, which means there is less confusion, translation and L1 interference.
In the classroom
Many teachers and learners regard expanding the target language lexicon - in other words, learning more vocabulary - as a priority. Advanced level learners, for example, will express the need for 'more words' frequently.