A level is a stage constructed by putting a variety of level elements together, which includes tiles, enemies, backgrounds, objects, warps and music and in this case is made using the Super Mario Flash series of games as well it's hacks.
The building blocks a level is made of, or simply said tiling, obviously plays a major part in the level. There are a lot of different ways to "tile" a level, as tiling can both be used to create challenges around it, but it also plays a major role in the visual department of the level. You can for example, mixing different tilesets together can create a whole new look.
Scenery is only used as visuals (as it can't create challenges), but it plays another major role in the visual part of a level. Scenery is often used to represent a level's theme, to add extra atmosphere to the level or just making the level look more visually stunning. You can also have Mario walk behind the scenery by putting scenery down in Layer 2 and then setting the default layer to 2.
In almost all cases, although all these elements of a level are important to have, gameplay is the most important part of a level. Good gameplay makes a level fun and prevents the level from being bland. Good gameplay examples are having for example a wide variety of challenges, unique/creative obstacles and fiar enemy placement.
Every level has a theme. Examples of themes are grasslands, desert, space and volcano. The themes are in almost all cases defined by the level's background and music, and the tilesets that come along with it. A level can have multiple themes as long as it's a proper theme transition (if it's a snow level and it has a cave themed bonus zone that's very small, it's still considered a snow themed level)