This is an online emulator designed to make it as easy as possible to get started programming the Motorola 6809 CPU in assembly language. There's the obligatory Hello World program ready for you to assemble (click the big green button marked Assemble...) and run (click the big green button marked Run (it's beneath all the dials and knobs to the left)). If all goes well you will see a greeting appear on the emulated text screen (the horrid-looking box below with all the green @ symbols).
If you're familiar with the 6809 hopefully the rest is pretty self-explanatory. There are a few short programs included to help you along (move your pointer to Examples).
If you know some other machine language than the 6809, then it should be easy to pick up using the documentation here. A reference book wouldn't go amiss though.
Even if you know nothing of machine language, try having a go anyway. A grounding in some other programming language will certainly help. Along with one or more reference books or instructional vidoes. But seriously, the best introduction to assembly language I've seen on Youtube is Computerphile:- Game Physics (in Assembler), which uses the 68000 CPU.