Complex sentence: Since a guitar string has two fixed ends, it will act like a standing wave, and therefore when agitated by either being plucked or strummed; the longest wavelength that the string can produce is twice the length of the string.
Split into simpler sentences: Since a guitar string has two fixed ends it will behave as a standing wave when agitated by being either plucked or strummed. The longest wavelength that the string can produce is twice the length of the string.