What is the RPE chart?

To put it simply, it is a chart that show you at what %1RM you should be able to perform certain lifts. Of course you could just search for a random rpe chart and it should be acceptable  but why settle for less when you can get a chart made just for you! Stoopidly cool if you ask me...

Why is custom chart better than any other?

For the same reason you would want a tailored suit instead of randomly chosen one by your grandma. Both are suits but only one is suited for you, of course there is a chance that you will randomly find that perfect one but save taking the risk for the roulette.  

What should I know?

The only thing you need to know are your recent 1RM/4RM/8RM , it would be best if you would test them in a span of week to know exactly what are you standing on at this time, I recommend doing it for squat, bench and deadlift separately since the % of 1RM might be different for every lift. After a week (or you can just take a wild guesses) you take the data,  gently put them inside the calculator below and it will show you the results. And for all freaks in the sheets there is an option for you to download the chart as a .xlsx. showing the % for any RIR above 5 for up to 12 reps.