ssh - do i need it and where do i get it?

Hi

I'm a newbie to the iphone world and I was wondering where you get ssh? I've looked in installer but doesn't seem to be there. When I do a search, the page comes back blank and it doesn't seem to be in any of the folders. I have a mac, so is the right thing to use? And, apologies for showing my ignorance, but is it really necessary to have it?

Thanks in advance
D x