Tagging means writing your name or a symbol on something, like a wall or a train, usually using spray paint. It can also mean adding labels or keywords to something to organize or identify it.
Tagging can be seen as a form of art or as a form of vandalism, depending on where it is done. It can also be seen as a way to organize and find things easily.
Examples:
He is tagging his name on the wall.
Tagging the train is not allowed.
They are tagging the boxes with labels.
We are tagging the wall with our names.
Tagging the wall with spray paint is not a good idea.
I saw some cool tagging on the train yesterday.