As a rookie, you get a mentor to help you learn all the ropes of NBA life. It's an experienced player on the team who's there to give you advice on finances, professionalism, lifestyle choices - on all the changes you are going through from being a kid in college playing for an education (remember, there is no money in the NCAA, they are student-athletes) to making six to seven-figure amounts. To thank them for that advice, you provide service to the veteran.
As in, you're their personal assistant. Pick up their dry cleaning, make sure their favorite food is always there for them, go get them something from the store at 2 am - whatever they ask, you are to do. This sort of initiation exists everywhere, but depending on the veteran you get, your experience may vary.
Kendrick Perkins was drafted to the Celtics, and he was "fortunate" to get Gary Payton as his mentor. The Glove is a tough-love kind of guy, and Perk had to learn it the hard way. This episode involved a 2 am Patron run.
The team plane landed and Payton felt like drinking Patron, so Perk was up to go and get it. The problem there was that Perk was only 18 years old, so getting alcohol after midnight for an 18-year-old was quite the challenge. Perk told Payton he probably won't be able to get it but Gary had a solution "Go to the club." (via ESPN)
So I gotta come out $500 to get in the club, cause I'm 18 so they were hesitant to let me in. On top of that, that's to get in, now I gotta talk to the owner now I gotta spend a $1000 just to get a bottle of Patron for him to let me leave the club with to bring to Gary Payton.
Veterans won't give you $10 if you're getting them a cup of coffee, but most will give the rookie money for more expensive endeavors. Remember, rookies scale contracts weren't all that lucrative once you take out taxes. Perk was down 1500 for this bottle, and when he came to Payton's door to give him the bottle all he got was "I appreciate you young fella" and the door slammed in his face.
What did Payton need a bottle of Patron at 3 am? We will never find out, but you can probably guess.