A silylium ion is a silicon cation with only three bonds and a positive charge.  The abstraction of the silylium ion is seen from the ruthenium complex shown below.
In the first step of this mechanism one of the acetonitrile groups is replaced by a silicon molecule where the bond between the silicon and the hydrogen is coordinating to the ruthenium.  In the second step a ketone is added for the nucleophilic abstraction of the silylium ion and the hydrogen is left on the metal.