What is 'Knit over Knit, Purl over Purl' ????

Hello, I have a men's pattern for a jacket
There is a pattern called 'fancy rib' in the jacket
Also one row says ' knit over knit, purl over purl'
What does that mean and how do I do it??
I do hope someone will know and let me know how to do it
I want to knit a Xmas present for our son in law

Thank you