What does "leveling up" mean?
What does "leveling up" mean? And why not spell it "levelling"? Even then it doesn't make any sense.
Each level has it own xp's. When you reach these xp's, you reach that level. For each next level is the xp's needed higher: the first only 60 xp's, the last thousands xp's. The maximum level is 25.
DL is based on earning xp's , leveling up with xp's and keeping a long streak of days on DL without interruption.
The xp's are points you can earn with a correct translation in strengthen skills, or when you translate all the sentences in normal translation mode.
But you still decline to explain  what "leveling up" means and  what xps mean. Why would I want to earn xps? Of what use are they and what does xp stand for?
A "level" is the number that represents your overall skill. The higher the number, the more skilled you are in that language. It all comes from MMO games like Dungeons & Dragons or World of Warcraft, in which your character gains levels in order to become stronger. It doesn't really have any use and it's there merely to compare your progress to others and motivate you to go further until you reach 25, which is the maximum level one can obtain, as louis.vang explained.
You gain levels by earning xp. XP stands for Experience Points. You earn experience points by doing excercise and strenghtening your skills. When you collect enough xp, you gain a level, for example:
Level 2 - 20 XP Level 3 - 50 XP Level 4 - 100 XP etc.
This is NOT the exact number of XP you need to get these levels on Duolingo, I'm just giving an example. If you want to see how many xp (experience points) you need to get to your next level, open your Profile Page (hover over your name in the upper right, then click Your Profile) and it will say on the right. Hope this helps.
Thank you for your explanation. I am totally unaware of these children's computer games.
@LINHARS: in the windows version you should see your achievements (XP's and needed xp's for the next level) under your name, under profile. Only in the Dutch>English course i don't see that information, but that's maybe because i reached level 25.
Some people have 100,000 their names are listed. Also I have come across people that think that they come to the end after one round. Keep repeating it. It's gets more difficult.