what does Yo mean?

It's a word Goober picked up in the Navy.

'i' in spanish

a baseball term short for "you're out"

something you show grade 1 kids before you catch their cheek

[youtube=https://www.youtube.com/watch?v=b2F5qf7kLqE]

Used as a greeting or to attract attention.
Generally.
The Urban dictionary defines it as shorthand for the word your.
The dictionary of slang defines Yo as an interjection used to get somebody's attention.