XP or 'experience points' are what you spend to increase your stats and skills to show your characters learned progress towards the things they want to know.
You can see how much xp you have when you type 'i' to see your inventory. The first line will look something like this:
You currently have 81 xp and 182 ap.
That would mean you have 81 xp to spend. Before you spend it, you want to know how much your next increase in stat or skill will cost though! And to find out, you would use:
xp/cost <skill name> or xp/cost <stat name>
If you have enough xp, you buy it with:
xp/spend <stat> or xp/spend <skill>
If someone has a higher rank than you do in a skill, they might be able to train you and reduce your xp cost. But, people can only train up to the level just below them. (If someone has a 5 in a skill or stat, they can help you train from 3 to 4).
If someone does not have the teaching skill purchased for themselves, they can only train one person a week. But, at rank 3 and up they can train more people. (Someone with Teaching 3 can train 2 people per week).
In addition, the more skill in Teaching someone has, the higher a discount they can help you get against your xp cost:
0 - 20%
1 - 25%
2 - 30%
3 - 35%
4 - 40%
5 - 50%
Remember that ANY discount is a good discount though! And if you want to train someone to help them lower their xp cost, use 'help train' in game to see the commands.