Much like 210; 515
is a "code-name" for Eating Out
a girl. 5 represents the fifth letter of the alphabet: E; 15 represents the 15th letter of the alphabet: O.
The actual act of eating a girl out is when some one goes down on a girl and licks/sucks/nibbles varied parts of her pussy; usually to make her cum.
Many claim this to be a delicious "meal"; hence the term "eating out".