ritenuto is Italian, which means slow down. In music, "slow down" means to slow down gradually.

The reason why a bar is added between each syllable in the score is probably because the word needs to be pulled very long in the score to tell you that the gradual slowing down process is going to last for a long range from left to right (exactly where it starts to slow down and where it lasts depends on where the mark is pulled in the score).

if you have any questions, please ask me!