The main plot of the novel drives the story forward through the whole work.
Some novels have only one plot. A simple romance's plot is boy and girl meet, one or both screws it up because of some inner flaw or weakness, but they manage to change enough to create a happily ever after.
Other novels have a major story line and minor story lines. Most often, these books mix genres like romantic suspense, or they are more complex in both subject matter and word count.
A minor story line is called a subplot. The two major types of subplot are the parallel and the independent subplot.
The parallel subplot is a smaller element of the overall plot that intersects the major plot with both its major character or characters and the events. The main plot affects the subplot, and the subplot affects the main plot.
In AVATAR, Sully's romance with Neytiri is one of the parallel subplots in the main story of Sully's learning about the planet Pandora and his decision to save it from the other humans.
His relationship with Neytiri is his personal introduction to the planet, its people, and their ways, and his emotional/romantic relationship with her teaches him the value of its people as well as giving him the original impetus to reconsider his decision to spy on the scientists and betray the locals to the corporation and its mercenaries.
In my STAR-CROSSED, Kellen's struggle against sexual slavery, his owner Cadaran, and his search for his freedom parallels Tristan and Mara's sweet relationship and their own fight for Tristan's freedom against Cadaran as representative of the corrupt government.
A complex novel may have numerous parallel subplots. Some may be almost as complex as the main plot, and others may be short and simple pieces of the puzzle that is the story.
A simple subplot in STAR-CROSSED involves Tristan's relationship with Floppy, the intelligent alien kitty.
When Tristan lives in Mara's house, Floppy sees him as a rival for Mara's time and attention, and the housekeeper has told Floppy that Tristan with his sneaky male ways is a danger to her.
Floppy works to prevent a physical relationship between Mara and Tristan, and he's more than willing to kill Tristan to protect Mara.
Floppy and Tristan gradually learn to like each other when Tristan teaches Floppy to read.
After Tristan saves Mara's life at the risk to his own freedom, Floppy is totally won over to Tristan's side.
This subplot not only drives the main story forward by interfering with the romantic relationship of the hero and heroine, it also is comic or scary in contrast to the main story line's tone at that moment to add variety.
An independent subplot doesn't impact the main story. A common use of this kind of subplot is in a mystery where the main character has a home life subplot as well as trying to catch the killer in the main plot.
At its least, an independent subplot gives a fuller picture of the main character or a more complete view of the world he inhabits.
At its best, it reflects the main plot thematically or emotionally. For example, the hero must face the death of his father and their issues of abuse at the same time as he is chasing a serial killer who targets elderly men which may indicate he was abused by an older man when he was little.
The TV show, HOUSE, often uses the independent subplot which involves the relationships of the hospital staff to reflect the main plot of discovering what is killing their patient.
In most episodes, House will gain a valuable clue to the illness through his interactions with another character during that subplot.
The very strongest subplot, even those that aren't parallel, brings a thematic, characterization, and worldbuilding depth to the novel.