any non-trivial digital circuit is usually clocked logic.
digital processors are always clocked. clock frequency determines how fast processor can run (how many instructions per second it can execute).
When you look into a chip that says 800MHz it means it can work at speed (clock) up to 800MHz. you can use lower clock (say 500 or 600MHz) if you like but trying to go faster than 800MHz - you are on your own. every chip will have some tolerance. of chip is rated 800MHz then every sample you buy will be able to run at 80MHz. you may try to clock individual samples faster (this is called overclocking) but success rate will vary greatly from sample to sample (and from product to product).
64-bit indicates width of data bus. the wider the data bus, the bigger chunk of data can be processed by single instruction.
256MB is amount of RAM or working memory. it is 256*1024*1024=2680160256 bytes, each byte is 8bits. one bit is either 0 or 1 (or true and false)