What does each permission in the permission set do?

  Permssion Info
Simpplr User Add On    
      Create Topics Create new topics by assigning them to feed items/ Simpplr Contents.
  Assign Topics Assign / Remove existing topics to feed items/ Simpplr Contents.
  Chatter Internal User Use all Chatter features.   
      Create and Own New Chatter Groups/ Simpplr Sites Create and own new Chatter groups/ Simpplr Sites
  Create and Share Content Deliveries for Chatter Files Share a Chatter file with Anyone by creating a file link.
Moderate User Add On    
      Manage Chatter Messages Access all users' messages (one to one) sent in Chatter.
  Manage Unlisted Groups View and moderate unlisted Chatter groups/ Simpplr Sites
  Moderate Chatter Manage Simpplr App features, manage all simpplr sites except private sites (without being a manager) and unlisted sites, edit/ delete anybody's feed comment
      Edit Topics Edit topic names and descriptions.
  Delete Topics Delete topics and remove all corresponding topic assignments from feed items/ Simpplr Contents.
  Merge Topics Merge one or more topics.
  Manage Public Document Upload logo
Additional permissions    
  View All Data Access Developer Console and execute code from there.. comes in handy for mass adding / removing users to/ from a group
  Author Apex Access Developer Console and execute code from there.. comes in handy for mass adding / removing users to/ from a group
  Customize Application Allows user to edit Simpplr App Org level settings, else you wont see this tab at all / Full admin rights within the intranet
  Download AppExchange Packages Allow user to install/ update Simpplr App into their Salesforce org/ Deploying new releases/patches
  Manage Package Licenses To manage licenses for Simpplr App/ Adding simpplr licence
  Assign Permission Sets Assign permission sets to users.
  Modify All Data Manage Private sites as well but not unlisted sites