Sometimes corporate securities are designed to convert from their initial form to another form at a later date under certain conditions.
At the early stages (as well as for bridge rounds), a convertible note (aka convertible debt) is designed to convert from a loan into an equity security. Sometimes that security will be a convertible preferred share, which is designed to convert into a common share under certain conditions.
Very large publicly traded corporations sometimes issue “convertibles” which are often debt (bonds) designed to convert to shares (usually common) at a pre-stated price as a means to make those securities more attractive (conversion is a “sweetener” or upside potential if the publicly traded shares rise in value).
In no case are convertible securities as fun as convertible automobiles.