Reach: Is the sum of all original author's follower counts (minus duplicate authors).
Spread: Is the sum of all secondary (retweets) author's follower counts.
If I post 10 times during a time period, I am the only one that posts, and I have 100 followers, the reach would be 100. (I was the original author, and we minus duplicate authors).
If you post 10 times during a time period, I retweet one of your posts, I have 100 followers, and you have 589. The reach would be 589 (your follower count since you are the original author), and the spread would be 100 (my follower count since I am a secondary author).