IIRC is a short written form of “if I remember correctly” or “if I recall correctly.” People type it in email, chat, and comments when they want to offer a fact but will not swear their memory is perfect. It is a small, polite hedge: I think this is right, though I might have a detail wrong. You will almost never see it in a school essay or a formal letter. People usually do not say the four letters out loud; they just write them. Though actually, both “remember” and “recall” are used for the R, and nobody is marking you wrong for either. It belongs to internet English, where a little uncertainty is often kinder than a loud claim.