Muxes are locations at which services can be found. On traditional networks (DVB-C, -T and -S), these are carrier signals on which the individual channels are multiplexed, hence the name. However, Tvheadend also uses the term 'mux' to describe a source for multiple IP streams - so an IP address, in effect.

The columns have the following functions:

Play
  Downloads a playlist file (M3U or XSPF, depending on your startup options) so you can tune to the mux from an external application.
Enabled
  Whether or not the mux is enabled and thus available.
EPG
  EPG scan setup
  Disable
    Disable the EPG scan
  Enable (auto)
    Enable the EPG scan (when some services from this mux are assigned to channels)
  Force (auto)
    Force the EPG scan (everytime when the EPG scan is triggered)
  Only EIT
    Do only EIT EPG scan (when some services from this mux are assigned to channels)
  Only UK Freesat
    Do only UK Freesat EPG scan (when some services from this mux are assigned to channels)
  Only UK Freeview
    Do only UK Freeview EPG scan (when some services from this mux are assigned to channels)
  Only Viasat Baltic
    Do only Viasat Baltic EPG scan (when some services from this mux are assigned to channels)
  Only OpenTV Sky UK
    Do only OpenTV Sky UK EPG scan (the TSID must match in the skyuk configuration file)
  Only OpenTV Sky Italia
    Do only OpenTV Sky Italia EPG scan (the TSID must match in the skyit configuration file)
  Only OpenTV Sky Ausat
    Do only OpenTV Sky Ausat EPG scan (the TSID must match in the ausat configuration file)
Network
  The name of the network to which the mux belongs. Networks are defined in Configuration -> DVB Inputs -> Networks - DVB-S or ATSC, for example.
Original Network ID
  The ONID is a code which uniquely identifies a network.
Transport Stream ID
  The TSID is a code which identifies the network origination source or transmitter.
CRID Authority
  The group responsible for the Content Reference Identifier for this mux. This is effectively a URL/URI to an authoritative source for programme data (e.g. series link).
Scan Status
  Whether the mux is being scanned (ACTIVE), is planned to be scanned (PEND) or has reverted to idle scan (IDLE).
Scan Result
  Whether there were any problems with the last scan of this mux.
URL
  Mux URL.
  udp://
    Raw MPEG-TS UDP packets
  rtp://
    MPEG-TS UDP packets with RTP header
  http://
    HTTP stream (MPEG-TS)
  https://
    Secure HTTP stream (MPEG-TS)
  pipe://
    Read standard output from an external program. If the program name does not have the first character '/', the PATH environment variable is used to find the program name in all directories specified by PATH. Additional arguments may be separated using spaces. A raw MPEG-TS stream is expected. The string ${service_name} is substituted with the service name field contents. The \ (backslash) character means "take the next character asis" (usually space or the backslash itself.
# Services
  The number of services found on this mux.
Character Set
  The character encoding for this mux (e.g. UTF-8).
PMT Descriptor 0x06 = AC-3
  Whether or not the empty PMT descriptor 0x06 defaults to the AC-3 stream. Some Chinese cable providers are using this. If unsure, keep this off.
Interface
  IPTV : the network interface/card on which the IPTV source can be found.
ATSC
  whether or not this is an ATSC IPTV source.
Priority
  IPTV : The mux priority value (higher value = higher priority to use services from this mux). Value 0 means use the IPTV network priority value.
Streaming Priority
  IPTV : The mux priority value for streamed channels through HTTP or HTSP (higher value = higher priority to use services from this mux). Value 0 means use the standard streaming network priority value.
Environment (pipe)
  IPTV : List of environment variables for pipe (like PATH=/bin:/sbin) separated by spaces. The backslash character is handled like in URL.
Respawn (pipe)
  IPTV : Respawn the executed process when it dies.