Security Token Offerings are a way of moving capital and money market securities onto the blockchain, with the advantages of streamlined custodianship, compliance etc.
The Ethereum protocol uses the ERC-20 protocol which allows for peer-to-peer trading, then you have something like securize's DS-Protocol, which covers lifecycle management of the tokens.
Basically, instead of spending a lot of money for IPO on the primary market, you can construct an STO, which allows business to fund projects, sell off equity stakes, issue debt etc at a fraction of the price eventually (EDIT)
All of which will be done on Blockchain