Codecs
A codec is a program that is used to encode and decode a video file. Many codecs are used to encode MP4 files, the most popular being DivX, Xvid and H.264. The Playstation 3 supports only the H.264 codec, so if an MP4 was encoded using any other codec, it will not play on the system.
Identification
If you do not know which codec was used to create the MP4 you want to play, there are programs that can identify a file's codec. GSpot, Codec Installer and MediaInfo are three such programs that can identify the codec a file was encoded with.
Solutions
If you have an MP4 that was encoded with a format that the PS3 does not support, you can convert it to an H.264 file. FormatFactory, Super and MediaCoder are all conversion programs that support the H.264 format.