62% in this case refers to the total amount of content (missions, etc.) that you have completed out of the total available.

Basically, you have completed the main storyline but not every possible side quest.

Check out http://www.gta4.tv/100percent.php for more information on everything that is avaliable.
Source(s):
http://www.gta4.tv/100percent.php

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We built a page on 100% percent completion for Grand Theft Auto IV when it first came out. Here's all the steps you would need to take in order to get 100% completion:

1. Complete 90 Story and Procedural missions: 68%
2. 30 car thefts: 2%
3. 50 stunt jumps: 2.5%
4. Collect 200 pigeons (flying rats): 2.5%
5. Locate random characters, complete their missions: 5%
6. Complete 20 crimes in vigilante: 2.5%
7. Kill all 30 people on the list: 2.5%
8. Beat friends or computers at each activity: 5%
9. Get all friend's special abilities and do all their activities (not including Dwayne or Roman): 10%

Here's our page in case you're interested.
http://www.mahalo.com/Grand_Theft_Auto_4_100_Percent
Source(s):
http://www.mahalo.com/Grand_Theft_Auto_4_100_Percent

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