In short, Rosencrantz and Guildenstern say that the king should be protected because there are so many who depend on him (and so many that would be affected if he were to die). Claudius has just announced his fear that Hamlet may do the king harm; therefore, he plans to send Hamlet (along with Rosencrantz and Guildenstern) to England. They are both willing to go. First, let's examine the specifics of what Guildenstern says:
Most holy and religious fear it is / To keep those many many bodies safe / That live and feed upon your Majesty. (3.3.8-10)
In other words, it is quite a feat to keep all of a king's subjects safe by attending to a king's well-being. That would have been enough. Rosencrantz could have consented with his simple agreement; however, instead Rosencrantz has something similar to say:
The single and peculiar life is bound / With all the strength and armor of the mind / To keep itself from noyance, but much more / That spirit upon whose weal depends and rests / The lives of many. (3.3.11-15)
In other words, every human is bound to keep himself from injury, but it's even more important to keep someone (the king) safe, upon whom so many depend. Then Rosencrantz goes further:
The cess of a majesty / Dies not alone, but like a gulf doth draw / What's near with it; or it is a massy wheel / Fixed on the summit of the highest mount, / To whose huge spokes ten thousand lesser things / Are mortised and adjoined, which when it falls, / Each small annexment, petty consequence, / Attends the boist'rous ruin. (3.3.15-22)
Here Rosencrantz uses two similes to describe the importance of a King, saying in essence that a king never dies alone. First, he compares a king to a whirlpool, pulling everything into it. Second, he compares a king to the center of a wheel, with the spokes reaching everyone around. When a king is ruined, everyone is ruined. And finally, Rosencrantz admits that "Never alone / Did the King sigh, but with a general groan" (3.3.22-23).
Thus, Rosencrantz and Guildenstern have managed to say the exact same thing five times. Repetition again and again and again and again and again. No wonder Claudius wants them in England "to stand it safe with us." Ha!