There's no getting around a consistently strong academic record if you're planning to apply to the best universities in the world. Yet grades and exam scores are not really indicative of a student's personality nor their character traits. That's where extracurricular activities can play a significant role in tipping the scales in your favor.
Universities consider extracurricular activities as those that are outside a student's regular coursework, such as athletic or artistic pursuits that students pursue on their own time. It's important to keep in mind, however, that Universities don't look for "well-rounded" individuals, but more someone who exhibits consistency and dedication in one or two extracurriculars.
It's nice to be able to write "skiing", "horse-riding" and "piano playing" as your extracurriculars, but what stands out more is if you won 1st prize in a national piano competition, or if you participated (and won a medal) in an international skiing tournament. It's easy to do lots of different things, but it requires extreme focus and dedication over several years to really excel at any activity. And those character traits - focus, dedication & excellence in whatever you do - are what admissions officials value very highly.
So if you love a particular sport, or have a certain musical talent, then do whatever you can to excel at it. Aside from the pure satisfaction of being one of the best sportspersons or musicans in your school, it may also be the decisive factor in helping you get into the university of your choice.