The quotation is from Guru Nanak, the founder of Sikhism—a monotheistic religion originating in the 15th century as an offshoot of Hinduism and Islam.
Guru Nanak was a mystic, which, in a religious context, is one who advocates the practice of pursuing and has the knowledge to themselves achieve a personal relationship (or "one-ness") with God. Mysticism concerns aspects of reality that lie beyond normal human perception, including experiencing the presence of and communion with God. As such, Guru Nanak naturally emphasized solitary meditation.
The specific quotation you've found simply reflects Guru Nanak's view that it is not possible to realize either God or one's self without the aid of a "guru", or experienced religious guide. Breaking it down piece by piece…
Let no man in the world live in delusion.
You should not be fooled into thinking something else than that which I'm about to say. The following point concerns an essential truth, and no one should allow themselves to be misguided into believing something different.
Without a Guru none can cross over to the other shore.
You need the aid of a guru (an experienced religious guide/teacher/master) who can teach and guide you. Without such a person, you nor anyone else will be able to achieve salvation or a relationship with God.
Basically, to put it in vulgar terms, Guru Nanak is saying that you've got to have a guru, or else you're going to be screwed (i.e., not be able to fulfill their ultimate religious destiny in the afterlife).
(I'm not an expert on Sikh beliefs regarding the afterlife. My research indicates that they believe in reincarnation, as opposed to a heavenly paradise. So in that context, the quotation probably means that you will be unable to be reborn.)
The teachings of Guru Nanaka live on in the Sikh religion. Just before his death in 1539, he appointed one of his followers as his successor, entrusted with carrying out the principles he had elucidated. These principles and responsibilities were them passed down through a long line of Sikhi gurus. Followers of Sikhism believe that all subsequent gurus possess the divinity and religious authority of Guru Nanak, and therefore guide one to both self and God.