TBSOTf is a very strong silylating agent, prepared by the addition of a tert-butyldimethylsilyl protecting group and a triflate, which is a very good leaving group.  It is used in the protection of ketones and alcohols by the addition of the silyl group to the oxygen atom. It is not widely used in most syntheses because it is expensive and does not typically store well.  However, it can be prepared by the reaction of tert-butyldimethylsilyl chloride and triflic acid.  TMSOTf, a similar compound with the tert-butyl group replaced by a methyl group, is much more common.