Krallen means "to claw" or "to clutch". It's a strong, visceral verb that paints a much more intense picture than simply "to hold".
In the song, Rammstein uses this word to create a startling and memorable image: "Sie müssen sich an Sterne krallen" (They must claw onto stars). Instead of peaceful celestial beings, this lyric portrays angels desperately clinging to the stars to keep from falling from heaven. This dark and poetic twist is what makes the word so powerful and enticing to learn.
Rammstein takes a common idea—that good people become angels when they die—and gives it a dark and lonely twist. The song starts by stating this belief: "Wer zu lebzeit gut auf erden / Wird nach dem tod ein engel werden" (Whoever is good on earth in their lifetime / Will become an angel after death). But instead of a peaceful paradise, the afterlife for these angels is terrifying.
The narrator, speaking as one of these angels, explains they are only visible "wenn die wolken schlafengehn" (when the clouds go to sleep). Their existence is not blissful. They are filled with fear and isolation, confessing, "Wir haben angst und sind allein" (We are afraid and are alone). The imagery gets even more desperate with the line "Sie müssen sich an sterne krallen / Damit sie nicht vom himmel fallen" (They have to cling to stars / So that they don't fall from the sky). It's a constant struggle just to hang on. It's no wonder the song's main message is a powerful rejection of this fate: "Gott weiß ich will kein engel sein" (God knows I don't want to be an angel).