"Topping charts" is a phrase that is used to mean that the song sung by the artist is at the number one position in some record chart. (A record chart is a ranking of recorded music according to popularity during a given period of time. Examples of music charts are the Hit parade, Hot 100 or Top 40.) Many different criteria are used in different charts, including sales of records, cassettes and compact discs, the amount of radio airplay, and (since the introduction of the technology) the number of downloads. The reason why "the" has not been included in the word is that it is not a single chart that is been talked about. There might be various charts that the artists have topped in their life time. So "topping charts" is a better way to say it.