I think there's a similar question to this. But I couldn't pick up the exact answer that i can fully hold on to. I became more curious after listening to 'Circle of Life' where the Chord progression went from Bb-Gmajor-Cm-Gb at the climax. It gives the emotional feeling to the song for me. So i just wanted to know what's it called. Because I think the VI should be a minor if we put it in the simplest of theory. Or am I dumb? I'm sorry but appreciate the help!
It's going from parallel keys. In key B♭ major, its parallel is B♭m. But take the relative minor of B♭ - G minor, and use that one's parallel, you find G major. Hence I and VI, rather than I and vi. That VI is also the dominant of ii (Cm), and actually moves that way, so V/ii - a secondary dominant chord in a key.
Moving between parallels is fairly commonplace, going back to Classical and Baroque, and even before.
To expand on Tim's answer: As well as being a secondary dominant (especially if followed by some form of II or ii or IV), it could be the start of a "chromatic mediant" modulation (if the key of G were to be confirmed.) [That didn't read much better after translating into German.]
If the harmony continues in the original key, it's a secondary dominant. If the harmony stays for some measures in the VI key (or a closely related key), it's the start of a modulation. The whole story may not be known until the end of the section of music containing the I-VI movement. That's one of the nice features of most musical gestures, they can have several different meanings.
In the case of this chord progression, the G major chord would not be called VI. Rather, it serves to lead to the ii chord — C minor. Specifically, it is the dominant, the V chord, relative to C minor: it's "V of ii", or V/ii. This is known as a secondary dominant, meaning that it's a dominant chord relative not to the main key but to a "secondary" one.