Suboids were the poor people of the planet Ix. They had a human body, but they did not exhibit typical human behaviour. The leaders of Ix, House Vernius, treated the suboids as "human robots", trained only for heavy jobs and duties.
The suboids worked in the deepest underground of the planet, where they labored building trans-spatial ships, such as Heighliners, which were eventually sold to the Spacing Guild. They had very little feelings, emotions, fears, creativity, as they were not encouraged to think for themselves.
The Tleilaxu, led by Hidar Fen Ajidica, took advantage of the suboids desire to not violate the prime commandment of the Orange Catholic Bible against producing Thinking Machines. For months prior to the Tleilaxu invasion of Ix, the Tleilax secretly stirred up rebellion amongst the suboids. This peaked with the full-scale invasion of Ix by the Tleilax, who were helped by the Emperor's [Sardaukar] troops.
Once Ajidica and the Tleilax conquered the planet and exiled House Vernius, the suboids were summarily placed back into complete submission by the Tleilaxu.