Ask HN: What does “Strong background in data structures and algorithms” mean?

Going through job descriptions of a 'Software Dev Engineer' role; I often see "Strong background in data structures and algorithms". I know this has probably been asked and discussed umpteen times before here. However , I am currently applying to companies , with 6 years experience as a .NET web developer , and cannot seem to understand the level of expertise expected when this is posted as a requirement.I do have basic knowledge of data structures and algorithms from my computer engineering days.I have worked with fairly large enterprise code bases. However , I face lot of challenges when trying to solve tech interview questions, whether on a phone screen, online or face to face interviews. So am trying to understand the level I need to achieve to become "Strong background in data structures and algorithms". I am not looking to pursue a MS/Masters degree or anything, but do want to achieve a level so I can gain the confidence to face tech interviews where this is a required criteria. Appreciate your thoughts and experiences.