Syncing with SalonInteractive means that a third party company will be controlling your product listings. They will control the name, photo, size, etc. and you wil not be able to make changes except to the price, on hand quantity, supplier minimum and order minimums. 

If any of your current products match a SKU that you select, it will permanently replace your original information. 

Sync New Product: The number of products that are new to they sync.

Replace Matching SKUs: If your current setup has a matching sku to a product selected, it will be permanently replaced with the SalonInteractive settings for that product. 

See example below.