A script that makes a rare accurance or appearance?

I've been wondering how you can make a rare appearance like it wouldn't be seen often script. I'm still stuck on it and can't find it out.

Your wording is a bit confusing. If you're trying to make a rare item, one that would appear randomly, but rarely, then there are several ways to approach this. You could have a 'pool', in which you have a table filled with different values. And of course, one of the values would represent 'rare', but only pop up once in the entire table. Then, just randomly select from it. Since you've shown 0 effort on attempting to script this, this is the best answer you'll get. Hope I helped!