I have often read that St. Thérèse of Lisieux suffered from spiritual dryness.” Can you please explain exactly what is “spiritual dryness.”

Spiritual dryness is just that: when it comes to conversation with God, you feel like you’ve got nothing to say and that God isn’t saying much either.  If you turned it into a movie, it would be a bust at the box office.  But from God’s perspective, spiritual dryness is a valuable test.  Those who persevere in their practices of piety and devotion – even when they feel they get nothing out of it – win God’s grace and grow in holiness by proving their fidelity and loyalty to the good Master.  
 
Not only did St. Therese experience this, trial, but many saints have as well.  There’s a marvelous section of the Catechism on prayer which is well worth reading.  Regarding spiritual dryness, it has this to say:

Another difficulty, especially for those who sincerely want to pray, is dryness. Dryness belongs to contemplative prayer when the heart is separated from God, with no taste for thoughts, memories, and feelings, even spiritual ones. This is the moment of sheer faith clinging faithfully to Jesus in his agony and in his tomb. "Unless a grain of wheat falls into the earth and dies, it remains alone; but if dies, it bears much fruit.” If dryness is due to the lack of roots, because the word has fallen on rocky soil, the battle requires conversion.(no. 2731)