Hi,
One example of proportion is 1:2=6:12 , with 1 and 12 being the extremes and 2 and 6 the means. In general, a proportion is an equality of two ratios, a:b=c:d, and a and d are called extremes while b and c are called means.
Hope this helps! If my answer was not clear enough or you’d like further explanation please let me know.