Why I can't play some video files?

The answer is simple, what you need is a codec interpreter. It is used to decode video files like a language interpreter so you can understand it well.

Technically:

A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "coder-decoder" or, less commonly, "compressor-decompressor". A codec (the program) should not be confused with a coding or compression format or standard – a format is a document (the standard), a way of storing data, while a codec is a program (an implementation) which can read or write such files. In practice, however, "codec" is sometimes used loosely to refer to formats.

A codec encodes a data stream or signal for transmission, storage or encryption, or decodes it for playback or editing. Codecs are used in videoconferencing, streaming media andvideo editing applications. A video camera's analog-to-digital converter (ADC) converts its analog signals into digital signals, which are then passed through a video compressor for digital transmission or storage. A receiving device then runs the signal through a video decompressor, then a digital-to-analog converter (DAC) for analog display. The term codec is also used as a generic name for a video conferencing unit. according to wikipedia definition

There are some free codec file you can use and the most common is K-lite Codec Pack and VLC.

K-Lite Codec Pack is a collection of codecs and related tools. Codecs are required to encode and/or decode (play) audio and video. The K-Lite Codec Pack is designed as a user-friendly solution for playing all your movie files. With the K-Lite Codec Pack you should be able to play 99% of all the movies that you download from the internet.

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VLC media player is a highly portable multimedia player for various audio and video formats as well as DVDs, VCDs, and various streaming protocols without external codec or program.
It can also be used as a server to stream in unicast or multicast in IPv4 or IPv6 on a high-bandwidth network.
VLC can play:

• MPEG-1, MPEG-2 and MPEG-4 / DivX files from a hard disk, a CD-ROM drive, and so on
• DVDs, VCDs, and Audio CDs
• From satellite cards (DVB-S)
• Several types of network streams: UDP/RTP Unicast, UDP/RTP Multicast, HTTP, RTSP, MMS, etc.
• From acquisition or encoding cards (on GNU/Linux and Windows only)

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