Q: my MAC start-up disk has no more space for application memory - what does this mean and what do I do?

It means you're in danger of loosing stuff on your Hard Drive, try to clear off a few GB of files & empty the Trash, then...

How much free space is on the HD, where has all the space gone?

OmniDiskSweeper is likely the easiest/best, and is now free...

http://www.omnigroup.com/applications/omnidisksweeper/download/

And if needed, you can use Terminal to run it in admin mode to see files/folders your user can't see...

http://www.macobserver.com/tmo/article/how_to_recover_missing_hard_drive_space/