Memblocks (memory blocks) are basically blocks of - you guessed it - memory.
It's an area within the computer's RAM used to read and write to at high pace. As such you can for example do very quick editing of images, sounds or meshes by manipulating their memblock data.
You can write anything to a memblock, as far as I know (certain things might have to be split up into bytes (or some other data types) first, such as strings though); the size of the memblock is dependant on how much data you need to write to it (or in case of a pre-existing memblock, how much data it contains).
A memblock mesh is a memory representation of a mesh, and as such holds information about each vertice's position in relation to the centre of the mesh, etc. The FVF format represents how the vertice data is stored in the memblock (the most common one in DBP is 338, which is documented in the help for
make mesh from memblock).
Finally, there are plenty of tutorials on memblocks around here, I believe. I don't know any particularily good one off the top of my head though.
Good luck learning - memblocks are
really useful once you get them
