Rachni are race of sapient, insect-like creatures with a eusocial organization. Rachni hives have a population of workers, warriors, and drones (also called "brood warriors") that follow the direction of a queen. They vary widely in size, with workers smaller than a human and queens as much as 17 meters in length. Rachni are strong, intelligent, and dangerous adversaries. In particular, they are capable of surviving in many environments that are toxic or otherwise intolerable to humanoids.
Rachni queens can transfer memories to their progeny, allowing them to maintain a sophisticated culture and technology base without a written language. Rachni can communicate telepathically, a form of contact that they call "singing". This form of communication is described in-universe as an organically evolved form of "quantum entanglement communicator". Rachni telepathy works with some other species, like asari, that the rachni describe as "receptive".
The rachni posed a threat to the galaxy roughly 2000 years ago, when the only members of the Citadel Council were the asari and salarians. Territorial and aggressive, the rachni began expanding when the mass relay in their system was accidently activated by the salarians, initiating the Rachni Wars. There are some indications that older races had been selectively breeding them for such aggression.
The salarians and asari did not have the numbers or durability to battle the rachni in their toxic homeworlds, so the salarians "uplifted" the krogan race to fight the war for them, giving them space flight technology so they could carry the fighting to the rachni homeworlds. The krogan were immensely successful, killing the rachni queens in their nests and bringing the race to the brink of extinction.
The rachni were technologically advanced enough to pose a major threat to the asari and salarians over a millennium before the Reaper invasion. There have been significant advances in technology and changes in military doctrine since that time, so it is unclear how the rachni at the height of their power would compare to modern Mass Effect militaries. The rachni homeworld and colonies had highly toxic atmospheres and their hives were placed deep into the ground, making orbital bombardment ineffective and ground attacks infeasible. The rachni also had a high reproduction rate. Ultimately, the reproduction rate and biological resilience of the krogan were required to defeat the rachni.
Rachni in the modern era of Mass Effect have only been rebuilding from one queen for a few years, and their numbers were again eroded by Reaper attacks, so their military threat level as a faction is insignificant. Direct transfer of knowledge from the queens to their descendants allows young rachni to be combat-effective without extensive training.