Anupraas/ Anupras (अनुप्रास) is basically repetition of letters, words or combination of letters in poetry. The most common form of Anupraas is words beginning with one letter repeatedly.
The most common example studied in textbooks is chaaru chandra ki chanchal kirane, khel rahi hain jal thal mein. Here, there are 3 words beginning in cha-, and three ending in -l. Hence, Anupraas.
But then, I think it'll be more interesting if I show you some examples from Bollywood.
One interesting use of Anupraas was tried by Prasoon Joshi in Fanaa, where he used it in the fun song (fun n fanaa could be anupraas too), chanda chamke cham-cham, cheekhe chaukanna chor, cheenti chaate cheeni, chatori cheeneekhor. Not an easy one where he puts twelve instances of cha, with all words starting from the same letter.
One simple use of anupraas was done by Papon in his own album's chhoti chhoti baatein hain ye, where there is a lovely line that goes 'seeli si saanson mein sehme sardiyaan, galti se phir karen hum wo galtiyaan.' Five out of six words in one line, with some lovely poetry.
By the way, Anupraas is something much like alliteration, though the range for the former is, I believe, a little wider.
Do you remember some more usage of Anupraas in Bollywood?