I have heard it a lot into recent times but I really do not understand what is it, Sous-Sol's album and some other projects has been referred as maxi single but what are those exactly?

In European/American terms, a maxi-single would be the equivalent to an EP. Longer than a regular single, but still shorter than an album. Say, somewhere between 13-33 minutes. In Congo however, a maxi-single is basically a regular-length album, but in the age of double/multi discs and hour-long albums, it is usually less than an hour. At least since 2000.