The Sunday School definition of a parable is often an earthly story with a heavenly meaning. This is true enough, but the mistake we often make in understanding parables is that Jesus told them in order to make difficult theological concepts simpler for broader audiences.
Like so many of our words, the word 'parable' is actually a Greek word that has descended down through the ages into English. You're likely familiar with the word hyperbole, which is a purposeful exaggeration (Example: I'm so hungry I could eat a horse). You also are familiar with the prefix para, which means alongside. A paramedic works alongside doctors, a paralegal alongside a lawyer, etc.
The root word is bole, which means to throw. So here's what we have:
hyper·bole = To throw over the top para·bole = To throw alongside
So parables are stories Jesus told that were alongside of the truth. If you're wondering why Jesus told so many 'alongside the truth' stories, you're in good company: the disciples wondered the very same thing in v10.
Jesus' response is stunning. He told the parables not to make things simpler, but to obfuscate! Here's His response:
"the knowledge of the secrets of the kingdom of heaven
has been given to you,
but not to them...
This is why I speak to them in parables... (v11-13a)"
Jesus quotes from Isaiah to bolster his point: