The wasteland has taught you some hard lessons, but you've remembered them all. You gain +1% to earned experience per level gained. (+26% experience at level 26, +27% at level 27, etc.)
Lessons Learned, much like Swift Learner, can help offset the penalty of the Skilled trait, allowing to cancel out the -10% XP gain entirely. It also has the benefit of requiring only one perk to match the effectiveness of all three levels of Swift Learner upon reaching level 30, though at the cost of having to take it much later. Like Swift Learner, its usefulness is debatable since there is more than enough available experience to reach level 50 without it.