Tiles are one of the more basic graphical "units" in ZQuest. Everything you see on the screen in a Zelda Classic quest is based on tiles, including the landscape, enemies, and Link.
A tile is a 16*16 block of pixels. The colors used for the tile are defined in a Cset, which means that each tile can use up to 16 colors (including one color that is used to define which parts of the tile should be transparent). Keep in mind that these colors are really only placeholders - you can draw a tile using whatever CSet you want to use, because later on when you assign that tile to a Combo, you can also assign the CSet.
Tiles versus Combos
A tile contains minimal graphical information. For instance, a tile might look like water, but there is no data stored with the tile that indicates to Zelda Classic that the tile should behave like water.
In contrast, a Combo does contain this extra information. When you create or edit a Combo you specify which tile it should use, along with how it should behave (e.g., it should behave like water, and it should be animated). You also specify which CSet the tile should be rendered in.