Well, in most RPGs, like Final Fantasy or Dragon Quest, you have dungeons--areas separate from the overworld map that have a specific connection to the story. The player goes there to complete a specific objective that advances the story, and the dungeon often ends with a boss. That would constitute a "level" in most RPGs. In fact, in the old Zelda games, the dungeons were called "Level 1, 2, 3," etc. So, in principle, you might say a "level" is a specific section/location of the game that tests the player's skill/growth thus far, whether it's through puzzles, battles, or some combination of the two. Each level might try to teach the player a new gameplay skill, and the boss (or whatever culminating challenge you want) tests the player's knowledge of that newfound skill or concept. Like, in Zelda's dungeons, each "level" is all about using a new tool you get in that level.
As other's have already said, a "level" is not necessarily "defined" in the RPG Maker genre. You can make levels whatever you want them to be! They can be the name of a location in the game. They can refer to your statuses and power "levels" in regards to abilities and magic. (Like how pokemon level up and go from Lv.1 to Lv.100).
It entirely depends on what you as the game maker choose.
A level could be pretty much anything. A level as in "character level" a level as in Difficulty level, a level as in space invaders... A level as in floor. There's so many things you can call level, you can probably write a book about just that.