What do the plus signs mean on the musical staff?

On this page: http://www.8notes.com/articles/instrument_ranges.asp It shows small plus signs near the ledger lines. What do they mean? Are they put where a new octave starts? For example, On a guitar, the notes on the fret board repeat twice. [e.g. it goes E>F>F#>G>G#>A>A#>B>C>C#>D>D#>E On the High [and low] E string. Then it repeats again in the same order. So do the plus signs show you to play the second group/octave of the same notes Or something else? Thanks. Hope I made sense.