[M T]

"/" is usually called slash.  "\" is usually called backslash. (For instance, see here)

The BB code used here employs markers in this fashion
   [FOO]text[/FOO]
That is a slash to indicate the termination of that BB code chunk.
(Since "FOO" isn't a BB code command, it just leaves the text in that line as it is, but if that had been "img" or "url" or "tweet", it would have caused something to happen.)

[M T]

"/" is usually called slash.  "\" is usually called backslash. (For instance, see here)

The BB code used here employs markers in this fashion
   [FOO]text[/FOO]
That is a slash to indicate the termination of that BB code chunk.
(Since "FOO" isn't a BB code command, it just leaves the text in that line as it is, but if that had been "img" or "url" or "tweet", it would have caused something to happen.)