Note: This is a spoiler free explanation! If you would like to know the true lore for the game (It is a very great story and I would 100% recommend it) Go ahead and play (Or watch a playthrough) Of Undertale! It is 10 USD on Steam.
The beginning of Undertale starts with a cutscene describing a war between humans and monsters, where when humans won monsters were banished to the underground. In the introduction, it also shows a child climbing up a mountain and falling into a hole leading to the underground. (See below)
After this, we wake up as Frisk, a child with yellow-ish skin, a blue shirt with two purple stripes, and dark brown hair on a bed of golden flowers. That is where we start our journey into the underground. After meeting a small golden-talking flower named Flowey, we meet a goat-monster named Toriel, who helps guide us through the start of the ruins.
After we leave the ruins, we encounter a skeleton named Sans, and his brother named Papyrus. we journey through a snowy forest, encountering many monsters until we reach a small town called Snowdin.
After that we head into Waterfall, a area with a dark and neon-blue color palette. This is where you meet the royal guard, Undyne.
After going past Waterfall we head into our 3rd main land area, Hotland. Here we find the Lab, where royal scientist Alphys and her creation Mettaton lives.
Going past Hotland leads us to the Core, where the entire underground is run. The Core is the power source of all things in the underground.
After this we head into New Home, a city very similar to the Ruins, however the color palette is dull and grey compaired to the Ruin's rich, purple colors. We head into the castle where the king of the underground lives, Asgore.
Depending on the route you choose (True Pacifist, Genocide, Neutral) This where will be the game either ends or continues after you fight Asgore.
The goal of Asgore is to collect 7 human souls in order to break the final room in the castle, the Barrier leading to the outside. So the question is, which route would you take?