'Severe ostial to mid vessel LAD disease' means this:
Ostia is Latin for 'mouth'. When you get an ablation for atrial fibrillation, the ostia of the pulmonary veins are isolated with a circle of tiny burns. That is, the mouths of each of the four pulmonary veins is encircled with small scars to block the unwanted electrical signals from emanating from out of the ostia of the four veins. This is what is meant by 'PVI' procedure, or simply 'pulmonary vein isolation.'
So, your report suggests that you have obstruction in your LAD (left anterior descending artery, also known as 'the widow maker'). Your obstruction is rated as 'severe', meaning you should be treated immediately. Further, the '...to-mid-vessel...' means it runs from the ostium, or the mouth, all the way to the midpoint of that vessel. So, it unfortunately is not good news.
Next, RCA is the 'right coronary artery'. The report is that the mid line, or point, to the far side is moderately diseased, so no real hurry, but something should be done about it soon...for your sake.
Here is an article on RFR and FFR pressure wire assessment. It's not in my wheelhouse as of yet, so you'll have to do this yourself. Note that there are a few click-throughs required before you will be allowed to read it properly:
https://www.cardiovascular.abbott/int/en/hcp/disease-management/physiology/indices/ffr-rfr-epicardial-assessment.html