Company A will accept funds to buy some stock like MSFT, then they will mint a token i.e. xMSFT that you'll own in a blockchain ledger. You can trade this for whatever you want but the company promises to buy any token xMSFT at MSFT price whenever you want[1]. So the tokenized asset remains pegged as long as there's trust in the institution.

Also, the company will grow a balance sheet out of the funds created by the owners of the tokens and they frequently will give you some sort of yield on your position.

[1] Open Market hours and usually accompanied by a fee.