Need to know what is the available physical memory means?

Available Physical memory is the part of your total RAM (256 MB in your case) that has not been allocated or is not being used by your computer, it means that the 56 MB free memory in your case is the availabe memory which can be used for other processes.

Hi Dear as our system starts then it loads operating system as well as various services to serve various request of the operating system and users, in to ram. Since thease services are the part of Operating System they could not be placed in Virtual Memory that is the reason with each os relase a minimum system requirement is specified. After Loading these servies and part of os the remaining portion of memory is used for applicaition management and virtual pages. that available portion is that u see in ur command. i think i made u clear if not then u feel relax to discuss more. thank you

Well the operating system as well as the other programs running on your system does uses the ram you have so when you are having 256 its shared between operating system and all programs running so the available physical memory is which remaines from your 256 after os+programs are deducted from 256.