This acronym stands for High frequency Active Auroral Research Program and refers to a US military operated research site in Alaska.
The official purpose of HAARP is to research the behavior of the ionosphere, a layer of charged particles (ions) surrounding the atmosphere. Large electric currents occur naturally in the ionosphere, specially in proximity to the poles, where particles captured from the solar wind are trapped inside the Earth's magnetic field. The HAARP transmitter can heat small portions of the ionosphere by focusing a beam of high frequency radio waves on a specific section of sky above the transmitter. This modifies the conductivity of the ionosphere and thus allows modulating the naturally occurring electric current. This modulation allows broadcasting ELF (extreme low frequency) radio waves all over the globe. The ionosphere itself acts like a huge ELF transmitting antenna.
The official uses of this are related to communication with submerged submarines and to imaging of underground structures. ELF waves travel well through water and ground and are thus suited for the purpose.
The HAARP installation is a common object of speculation. Various parties allege it is used for control of the weather, for mass mind control or for hyperdimensional applications such as travel between universes or densities.
The Cassiopaea material states that HAARP is to be used for space and time manipulation, such as passage between densities on the surface or underground. Mind control in either 3rd or 4th density is also mentioned.
ELF frequencies corresponding to the naturally occurring brain wave frequencies around 10 Hz or their harmonics can be used for affecting brain states. This would support the possible mind control applications of HAARP, since HAARP can transmit at around these frequencies.
Commenting on the trans-density applications of HAARP is not possible since we do not have a theory of these.
See Mind Control.