You can look up 'subplots' and how to use them but generally, a story has the main arc (the big plot, your biggest point/goal yada yada), while subplots are there to add depth, detail, and tension to everything from the characters to the whole story. It's not necessarily 'secondary' but it can be.
A 'romantic' subplot is then romance that is not at the core of the story. That doesn't make it unimportant, nor does it indicate harems. It's just a 'sub' to the main plot. Most stories/novels/television shows out there contain a 'romantic' subplot.