"...not on those days."
That's fine, we'd still love your help. If you have some time to spare before the formal sprint dates either leave a comment on the Join the 4.5 Documentation Dash blog indicating the issue you've chosen and a comment on the issue on Jira or claim it yourself if you have that level of Jira permissions. If you won't have any free time until after 28th July, don't claim an issue now; please leave a comment below and I will contact you if there are still issues left after the formal sprint ends. (Issue CRM-14635 will need to wait until all the other issues are completed, so is a likely candidate for post-spint work.)
"...I am very new to CiviCRM."
Maybe you don't feel you have enough experience to be writing documentation. Proof-reading text doesn't require in-depth knowledge and can be a great way to become more familiar with CiviCRM functionality You need to check for spelling and grammar mistakes (both English and American spelling are OK, so you don't need to change every 'organization' to 'organisation' or vice versa). You should look for repeated words and sentences that don't make sense or aren't finished. It would also be great if you ran through any steps on a 4.5 sandbox site (eg http://drupal.sandbox.civicrm.org/) to make sure they are correct and explained in enough detail. You correct the document as you read it, but if there parts you don't know how to fix add a comment below or email me (joanne.civicrm at gmail.com).
Proof-reading is our "In Quality Assurance" stage and should alwys be done by someone who didn't write the text. There are already seven issues waiting for the final proof-reading stage, and every issue from the sprint will need to go through it, so even if you are very new to CiviCRM you can help out.
"...I don't understand what I need to do."
The sprint board at https://issues.civicrm.org/jira/secure/RapidBoard.jspa?rapidView=14&view=detail is just a different way of displaying issues that already exist in Jira.
Looking from left to right:
To sign up for an issue, leave a comment on the Join the 4.5 Documentation Dash blog and a comment on the issue in Jira. Start work as soon as you want to.
Please help out if you can. CiviCRM is packed full of features; for new users there is a lot to learn and for experienced users there is a lot to remember. Having good documentation helps everyone and will help keep CiviCRM strong.