These two terms have varying definitions across ad platforms. Do check how the platform you are using defines these two metrics.
Generally, the definitions should be as such:
Reach is the total number of people who see your content.
Impressions are the number of times that your content is displayed, regardless if it was clicked.
Therefore, a single user can view your ad multiple times, and have multiple impressions. However, he/she will only count as one when it come to reach. Naturally, it is normal for impressions to be a larger number than reach.
When do I use reach?
Reach is used best when the goal is to acquire new customers. By tracking the number of unique users that viewed your ad, you will accurately gauge the number of potential interest in your product/service.
When do I use impressions?
Impressions are best used when the goal is to build your brand among existing customers. This allows you to track how familiar your existing customers are becoming to your brand.