Everything the processor can access is in memory. Memory is just a bunch of sequential locations. These locations can be RAM or ROM or even a piece of hardware. Each location is a byte wide and each location has a number or address associated with it. On the 68K the first address is 0 and the last is $FFFFFF giving it access to 16MB of data.
A location in memory can store data. Data is just a number. Technically this number is stored in binary, but binary is inconvenient to work with so usually hex is used instead. Any kind of data whether it be text, graphics or a web page is made up of numbers.
Instructions are a special kind of data. They are still numbers, but they serve a special purpose. They tell the processor what to do. Instructions are executed in order until a special kind of instruction is executed that tells the processor to start executing instructions somewhere else.
These numbers can be hard to remember, especially since there are a lot of them. To make things easier for programmers assembly language was invented. Assembly replaces numbers with easier to remember mnemonics so instead of having to remember $4E75 I can remember RTS instead. Don't worry about what this instruction does yet. We'll get to it later.