The Phrase is the fragment of text containing the term you have LingQed, taken from the lesson in which you found it.
If you import your own vocabulary, you'll need to include a phrase with each term. This way, when you do your flashcard review, there is a sentence associated with each term so you can fill in the blank.
You can also LingQ full phrases from lessons. It is recommendable to do this when you come across a commonly used phrase or an idiom.
If you want to translate a full sentence, you should use Sentence Mode. Do not make a habit out of highlighting and saving full sentences - just phrases of ~3-6 words.