Just like real life, your characters will have more than one thing demanding their time and attention. Romances, family life, work concerns, health issues, friendships, etc. These additional plot lines are subplots that give your story depth and help keep it moving. And as with your main plot, all subplots should follow a narrative arc of conflict, crisis, and resolution, usually wrapped up before the main plot’s climax. Subplots can be what’s happening to secondary characters or an internal conflict your main character is facing in addition to the main conflict of your story. The key to an effective subplot is how you work it into the main plot.
Plot is what happens to your main character (MC). Things happen and your MC has to deal with or resolve these issues: they receive a mysterious message, they come home to find their spouse in bed with someone else, their house burns down, etc. One thing happens, then another, then another, and each event leads your character further along your narrative arc toward the climax. Plot is what gives us action. The narrative arc, working in tandem with the character arc, gives us the reaction.