A cross-sectional study collects information from a group of people at one point in time, to describe how common something is or how two things relate.
A survey that asks adults about ADHD traits and about anxiety on the same day is cross-sectional. It shows what exists together at that moment.
It can't show what came first. If anxious adults score higher on an ADHD questionnaire, a cross-sectional study can't tell you whether the ADHD led to the anxiety, the anxiety affected the answers or something else explains both.
For questions about change over time, researchers use a longitudinal study instead. Cross-sectional studies are quicker and cheaper, which is why so many exist. They are often the first kind of study done on a new question.