What is the last column on the scoreboard before players' items?
Solution 1:
That is the accumulative total of Card XP (CXP) that that player has earned for your team.
According to this forum post, killing towers/inhibitors, and using harvesters will contribute to global CXP, increasing the total in the 5th column.
It doesn't include your individual CXP, earned from minions and enemy heroes, however. So that total is strictly the global CXP.