Legacy System Extension

That is easy, Mac OS Catalina and beyond will only run 64 bit apps. Your system is warning you that you may have some 32 bit apps that will not work once upgraded. To find out which they are, open System Information (Applications - Utilities) or open About this Mac and click the System Information link or use Spotlight to locate it. Then click Softare - Applications and your apps will be listed and then tell you which are 32 bit and which are 64 bit.

rkaufmann's advice was correct, but I think the message you received was addressing a different issue. Apple is now sending out advance notices that, in the future, their operating system will no longer be compatible with some programs which rely on Apple's system extensions. There is nothing we can or should do at this time. All software vendors have been notified of the coming change and the responsible ones will make the needed changes when needed. Those who do not will have software which will be incompatible. I hope and assume that when that time comes, we will be advised of some way to easily determine which programs are not updated. Then we can decide to either not upgrade our operating system at the time, do without those programs which are incompatible, or download compatible versions of the programs which will no longer work. For now, don't worry about it. I hope this is helpful for you.