The channels are listed / edited in a grid.

The columns have the following functions:

Number
Channel number. This is not used by Tvheadend internally, but rather intended to be used by HTSP clients for mapping to remote control buttons, presentation order, etc
Name
Name of the channel as presented in EPG and external interfaces. Changing name of a channel does not interfere with scheduled recordings etc.
XML-TV Source
Name of the XML-TV channel. If you have XML-TV enabled you can select which XML-TV channel to obtain data from. By default Tvheadend tries to match the name itself, but sometimes it might not match between the XML-TV source and the TV provider so in such cases you can change it yourself.
Tags
Each channel can be bound to a zero, one or many tags. The tags are used for multiple things and it's up to you how you wish to use and name them. Tags can be used to aid searching in the EPG if you have many channels. The tags are also presented in a Media player.
DVR Pre-Start
Allows the user to specify an amout of extra time that should be prepended to a recording scheduled on this channel. In other words, if you type 5 here for a recording that is scheduled for 20:00 will start to record at 19:55. This setting overrides the Extra time before recordings DVR setting.
DVR Post-End
Similar to DVR Pre-Start this allows the user to add an extra amount of time that should be appended to a recording. In other words, if you type 5 here for a recording that is scheduled to end 20:30 will stop to record at 20:35. This setting overrides the Extra time after recordings DVR setting.