Commission in the context of digital distribution means the percentage of revenue earned from your digital distribution that the distributor keeps.
A digital distributor is a company that distributes your music to various online music services like Spotify, Apple Music, Tidal etc. They often provide related services like cover song licensing and listing your music with a performance rights organisation.
ISRC stands for International Standard Recording Code. This is a unique identifier assigned to each and every song to help with rights management. Most distros will assign them automatical and some allow you to use your own ISRCs if you have them.
Lead Time typically refers to the amount of time a publication (print/online) needs between you contacting them for a review or coverage and your release date.
UPC stands for Universal Product Code. This is the barcode number of your release and every release has to have one. Some distros will charge you for them and some provide them for free.