Soft questions are those that don't require the use of actual physics problem-solving skills or knowledge of the state of physics research to answer. In other words, soft questions are those which are of interest to physicists but are not actually about physics. They may ask about some aspect of physics research or study, such as history, learning techniques, publication advice, etc. though do note that there are other tags for several of these areas, such as history and education.
Many soft questions are just on the border of being off topic for this site; accordingly, the definition of the soft-question tag has fluctuated somewhat as the scope of the site has changed. If your question fits under this tag, it might be something that is worth bringing up in chat.