Pine Script has single-line comments only. Any text from the symbol
// until the end of the line is considered as comment. An example:
study("Test")
// This line is a comment
a = close // This is also a comment
plot(a)
Script Editor has hotkeys for commenting/uncommenting block of code: Ctrl + / Thanks to this, there is no need for multi-line comments - one can highlight the large code fragment in Script Editor and press Ctrl + /. This will comment out highlighted fragment; if Ctrl + / is pressed again, commentary will be taken away.
Also, comments could not be placed in the middle of a statement wrapped to several lines, read more here: Lines wrapping.