HTDM stands for "Host TDM" and refers to Plug-Ins that do all audio processing on the host CPU instead of the DSP chips found on your ProTools TDM (Mix, HD, etc) hardware. They use a single, shared DSP on the TDM system to allow the audio from these host Plug-In processes to stream in or out of the system. The faster your computer, the more Plug-Ins you will be able to run.
HTDM Plug-Ins allow the power and flexibility of host-based processing for synths and sampling to directly integrate into Pro Tools. You can work with these Plug-Ins as you would use standard TDM Plug-Ins, including major benefits like using Pro Tools automation (in addition to MIDI) to automate parameters, or using control surfaces for hands-on control of Plug-In parameters.