Essentially it means the seller has NOT set a minimum amount of which theyβre happy to sell at, i.e. the domain WILL be sold (at auction) regardless of what the winning bid is
Reserve price provides you a shield in domain auctions.
Lets say you listed a domain on an auction platform with a reserve price of $100. You will be only obligated to sell the domain if the highest bidder's bid is >= to $100.
If the auction ends with a highest bid of $99, then you don't have to sell the domain. In short, you are making sure that you get at least $100 from auction.
No reserve = No shield
In "no reserve" there is no shield and if the highest bid is >= $1 when the auctions ends, you will have to sell the domain.
Some people like to use protection and some don't (when they are feeling adventurous ).