Sumittable (formerly Submishmash) is a program that allows an author to submit and multiple editors of the same publication to read and reply to that submission. It is not something you ever have to worry about unless you want to use it to accept submissions, or you want to submit somewhere that only accepts through their interface.
If you do want to submit somewhere and they use Submittable, you'll know it. The market points you there, then you have to create an account and submit through Submittable - the instructions are very clear at that time. Freelance writers report having to use it for almost 25% of their 2011 submissions.
When a piece is rejected or accepted, you can click on the story/poem title [in your account] and read the comments from the editor. One freelancer got a pleasant surprise when she clicked on a flash piece she'd submitted to a market a year or so ago.
"I was so discouraged after it was rejected, I deleted it. Come to find out, the editor read it repeatedly, thought it was wrong for his publication, but praised the story and gave me the names of three other flash venues to try. And here I'd been thinking the editors were a cold, haughty bunch, not even taking time for a polite 'sorry, but no'."You should get both submission acknowledgements and rejection and acceptance emails automatically because that's automated, unless you've opted out on the settings page.
Editorial comments should reach you together with the rejection. If they didn't, the editor chose not to send them (probably by mistake since "no reply" is a default in Submittable.)
Solution: click on the help button at upper right corner and ask for your settings to be checked. The help techies fix it.
Be aware that when Submittable receives a bounce for the reason "invalid email address" the system will not continue to send to a bounced address. Have the techs clear your email from the bounce list so that the system will start sending notifications to you again.
Submittable offers a free trial. Freelancers report its actually pretty handy. You may find it well worth opting for one of the Plans.
In addition to Submittable many writers also use Duotrope's tracker, and praise it highly.