This can refer to several things:
1) When the subject (usually yukkuri characters) of the picture is depicted to be interacting in a way as to be aware of their fictional nature (aka. "breaking the fourth wall"). Examples: Pictures depicting yukkuri directly addressing the reader (post #26174, post #21586, post #4225, etc.).
2) When the subject (usually yukkuri characters) of the picture is depicted to be interacting within as if they are in the real world. Similar but not quite the same as 1). Examples: Pictures depicting yukkuri using computers or laptops. Bonus points if they're using it to browse 2ch/OYP. (post #36263, post #18499, etc.)
3) Artist and/or author self-inserts, except that it is more in line with 2). Examples: post #34849, post #14445. A variant of this is where characters from other series interacting with each other. Examples: pool #196, post #22622.
The big difference between this and simply just adding the crossover tag is when the characters are aware of their fictional nature (the expression "going meta on us", so to speak). It the characters or artist/author avatars are not shown to be aware that they are in a work of fiction, the meta tag is unnecessary, and adding tags identifying them is enough.
tl;dr version - if it's breaking the 4th wall, it's meta.