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What does rRNA stand for? Well you probably know RNA is ribonucleic acid, but the 'r'? It stands for Ribosomal. So in particular, rRNA is ribosomal RNA and all that means is that it's the RNA that is a part of the ribosomes.
To understand what rRNA does, you have to know what ribosomes do and how they interact with the tRNA and mRNA.
Basically, rRNA is what makes up the large and small subunits of a ribosome, which is responsible for making proteins out of information sent from the DNA. DNA gets transcribed from the nucleus into mRNA (messenger RNA) that has the code for a protein. This code then runs through the ribosome where it meets with it's complementary tRNA (transfer RNA) bases and is translated into proteins.
I can elaborate on more of this or any other parts that seem fuzzy to you. It's a large overall process that involves a lot of interesting, tiny things inside our bodies that we never see in our normal daily lives!