Student or Learner
Don't be lost in thought and waste time.
Can you tell me the meaning of the highlighted parts?
Gary, it's not very natural.
To be "lost in thought" means that you are thinking so hard that you are not paying attention to your surroundings. If I am lost in thought at work and I am thinking about work-related things, I am not wasting time.
To "day dream" means to spend time thinking about unimportant things, letting your mind wander, or enjoying a bit of a fantasy (what you would do if you won the lotterly, what you want to say to your ex-boyfriend if you should ever see him again, etc.). If you spend you day at work day dreaming, you are wasting time.