Valonqar is High Valyrian for "little brother". The term is used by Maggy, a maegi and fortune teller from Lannisport, when making a prophecy to Cersei Lannister in 276 ᴀᴄ. The translation of the term is later given to Cersei by Septa Saranella.
And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you.
Since hearing the prophecy, Cersei Lannister has become convinced that her youngest brother, Tyrion, is the valonqar. This caused her to despise and mistrust him even more than she already did before.
The identity of Cersei's prophecized valonqar has been heavily debated among the fans of A Song of Ice and Fire and numerous suggestions have been made thus far.