Polarise is the British spelling of polarize. The word means to split people into two clear camps with opposite beliefs, or to make them split that way. A debate can polarise a town; a leader can polarise voters. The picture behind it is two poles, like the two ends of a magnet, pulling in opposite directions — though actually in everyday news it is almost never about the North Pole or about science. British papers and school books often keep the -ise; American ones write -ize. It is the same word. The matching noun is polarisation in British spelling and polarization in American spelling.