Pumped up kicks are expensive shoes. "Kicks" are slang for shoes since you kick with your feet and also wear shoes on your feet. "Pumped up" means high priced, high quality, and expensive. The term could also be referring to the wearer's ego being "pumped up." The song is a sort of threat to all the "popular kids" who typically have fancy shoes and base their opinions on others through what sort of materialistic things they own. "You better run, better run, outrun my gun" might be telling the "popular kids" to watch out because the singer has something even more worthy of glorifying than their fancy shoes. Also, keep in mind that most fancy shoes are not always good for running in.

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You can find the meaning here: http://es.urbandictionary.com/define.php?term=pumped+up+kicks