Dehydration Synthesis: Why does a monomer contribute a -H instead of an H atom?
The dash ("-") does not represent a negative charge or an electron.  It simply indicates that the group of atoms ( -OH or -H ) are attached (covalently bonded) to other atoms in a molecule.  So they are saying one monomer (molecule) contributes OH to the reaction and the other H.  (A charge is indicated by a trailing superscript minus sign.)