What does the second XP marker mean?
Solution 1:
That number is the total amount of trainer experience you've earned. So, in the image attached in the question, that trainer has earned 72,238 XP at the current trainer level, needs 100,000 XP to reach the next trainer level, and has accumulated 407,238 XP across all levels in total so far.
Solution 2:
Total experience accumulated throughout all levels.