( The two 'TH's in this name are pronounced like 'th'in 'with'. Also, see the link Thrigartha. )
Grammatically, Thrigarthas means 'the people of thrigartha'. But, contextually, this word means fighters belonging to or from the country of Thrigartha.
Thrigarthas was the collective name given to the king of Thrigartha land Susharma and his four brothers . In the Mahaabhaaratham war, the Thrigarthas mostly engaged Arhuna with their forces causing the Paandava side suffer his absence . That tact resulted in the killing of Arjuna's son Abhimanyu too . Finally, the Thrigartha forces were totally destroyed by Arjuna by the sixteenth day . The thrigartha forces were also known as samshapthakas(=ones under vows) due to their vows to engage Arjuna in the war continuously , especially on th thirteenth day when Drona planned for the Lotus formation..
The Thrigarthas also attacked Viraata on the southern end as a part of the Kouravas' strategy to find out Paandavas .
After the war, when Yudhishthira undertook performance of Ashvamedham, Arjuna went with the sacrificial horse for its protection. During that expedition, the Thrigarthas fought with him fiercely, with a grudge against him due to killling of their earlier king. Sooryavarma first fought with Arjuna. After the fall of Sooryavarma, his younger brother Kethuvarma led the attack. When Kethuvarma too was killed, a young Dhrithavarma personally attacked Arjuna. Even Arjuna's Gaandeevam fell off his hands due to the young man's attack. Arjuna admired his young opponent's skills. But, seeing all their army smashed by Arjuna, in the end, they surrendered to him .