Interfolio is a medium and a storage
You ask professors to write you a LOR
the website send an invitation for them, they should received it and write you something.
The website notify you that they did write you something and their letter will be stored there
Then you can later use it in your applications.
THe website keep your letter for a price
1 year for 16 I think or 20 dollar
3 years for another amount of money
but I don't think they ever delete it, the only difference between 3 years and 1 year is you can access your LOR for three years
just in case you needed to apply later to some other place.
Bad news is, you have to pay each time they send a lor for a school.
Good news is you see someone now, ask him for LOR, instead of asking him after a year when he forgot you,