Proximity searching allows you to find words that appear near each other. This can be used, for example, to find phrases that are similar (e.g., Perceived Stress Scale and Perceived Maternal Stress Scale) without doing multiple searches.
Increasing the number increases the maximum number of searchable words that can occur between your search terms.
For example, perceived NEAR/2 stress NEAR/2 scale retrieves both Perceived Stress Scale and Perceived Maternal Stress Scale. (Note that the word NEAR must be in capital letters.)