There are some words in English that are pronounced differently by different speakers. For example, some speakers pronounce the word economics with an initial [ɛ] - [ˌɛkəˈnɒmɪks] and others with an initial [i] - [ˌikənɒmɪks]. In this word, [ɛ] and [i] are said to be in free variation. Free variation can also be found when you hear British English speaker says the word tomato [təˈmeɪtoʊ] ,while North American speaker says [təˈmɑtoʊ]. Another example is the glottal stop in the word button. You might hear one speaker pronounces it [bʌtn] while other speaker pronounces [bʌ?n].
In linguistics, free variation is the phenomenon of two (or more) sounds or forms appearing in the same environment without a change in meaning and without being considered incorrect by native speakers. However, as for the case in the sounds [i] and [ɛ], we cannot substitute those two sounds in all words. Did you beat the drum? does not mean the same thing as Did you bet the drum?.