I heard I have to pull my foreskin back before I put a condom on. Is that true? And why is that?

What you heard is correct. When you put on a condom a little extra space should be left at the tip to catch the ejaculate. If you don’t pull back your foreskin before putting the condom on, it may roll back during intercourse. Your glans will then fill the empty space at the tip, which increases the risk of the condom tearing. Another advantage of pulling your foreskin back is that your penis can move unrestricted.