That is the ratio between the number of letters a user sent and received.
For instance, Ratio 1:1 means, on average, the user will send out 1 letter when receive 1 letter - a very responsive user; Ratio 8:10 means the user will send out 8 letters when receive 10 on average - less responsive but still quite active.
Please note that the ratio number is NOT the total number of letters, i.e. Ratio 3:1 could mean the user sent 31 and received 12. These numbers have been rounded up to a simple ratio.
It could be a reference of a user's responsiveness that give other users know what to expect before adding them as new friends.