Taking after the official Lua C API documentation, each C function that operates on Lua values has an indicator like this:

[-o, +p, x]

The first field indicates how many objects the function removes from the stack; the second field indicates how many objects it pushes to the stack; and the third field indicates what kinds of errors, if any the function may throw. A '-' means the function will never throw an error, a 'm' means the function will only throw in cases of memory errors, 'e' means the function may throw other kinds of errors, and 'v' means the function may throw an error on purpose. 'e' and 'v' error fields are usually documented more thoroughly in the detailed description.