http://niklabs.com/

What is playlist and where can I get it?

Playlist is a file that contains info about channels and videos. Such as channel name, stream URL, logo URL, matching EPG channel name, etc.
Supported playlist formats are M3U and XSPF.
You can ask playlist URL either from your local internet provider (if he provides IPTV service) or find free/paid playlists on internet.
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Note: there are plenty of sites offering this service on Internet, just check some. (soon here 10 IPTV providers)

All or some of my channels do not play or have playback issues (Android)

Select different Decoder type in Settings (Playback->Decoder).
If you see notification message, the app must be restarted to apply changes.

What are the different Decoder types? (Android)

Settings->Playback->Decoder

Auto: Aims to find the most suitable decoder for a given stream/file.
Native: Uses decoding abilities implemented in device firmware.
Hardware: Software implementation with hardware video decoding on supported devices.
Software: Completely software implementation.
HW+: Supports only hardware accelerated decoding. Has teletext subtitles support.