Text dependent question are questions that can only be answered by looking at/understanding the text.
For example, think of the story Charlotte’s Web by E.B. White.
A text dependent question would be: What were the reasons Fern gave to her father for not killing Wilbur?
A non-text dependent question would be: Have you ever stood up for something you believed in and why?
Both questions are good, but only one requires the text in order to answer the question.
Think about the questions you ask. Be sure to include text dependent questions, as they require students to show a good understanding/comprehension of what they have read.