DDE is "Dynamic Data Exchange", an older intra-application communications protocol. Excel supports communication with other applications via DDE. DDE is for sending variables and instructions between applications (in this case, application 1 is your instance of Excel, and application 2 is the broker's API application).
The fact that multiple broker's APIs support DDE may make it somewhat easier to port your application to another broker's API, however, the issue you need to focus on is that the broker's APIs are going to be different and therefore the formatting of the instructions that you send to each via DDE will be different. So, if you have developed a connection to one broker via DDE, you'll have a head start in developing a connection to another broker, but that's all: a head start.
If I remember correctly, I first wrote applications that used DDE back in 1992. It's OLD. While it's not extinct yet, I would not trust my money to trading over DDE. I believe it's subject to latency issues. Newer versions of Microsoft Windows and Excel use ActiveX to communicate with each other. I'd go with an ActiveX connection for most systems that don't require ultra high frequency. Hope this helps.