For those who don't know Jango Fett's backstory and who Jaester Mereel here's some history :
For many millenias the Mandalorians had been one of the most important factions and the best mercenaries and hunters of the galaxy, though they were divided in several clans that all answered to the leader of the mandalorians named Mandalore.
Several decades before the Clone Wars a mandalorian native to the planet Concord Dawn became the new Mandalore, his name was Jaster Mereel and he was once a brutal and ruthless man who had underwent a honorable change and had decided to reinforce the Mandalorians' code of honor by bringing new rules such as forbidding the murder and execution of harmless women and children or war of conquests. However Jaster has a rival who was not happy with his ascension and with the new rules he brought to the Mandalorians.
His name was Tor Vizzla and he incarnated in many ways the worst aspects of the Mandalorians : barbaric, ambitious, hungry for conquests, cruel, perfidious, etc..
Instead of obeying his old rival Vizzla rebelled against Mereel alongside those who opposed Mereel's reforms and created his own group that he called the Death Watch.
A bloody and long civil war ensued with the True Mandalorians fighting the Death Watch for years. During this war Vizzla led an attack against Mereel's homeworld Concord Dawn, killing the parents of a young boy that Jaster Mereel would find and adopt as his own son. This boy was none other than Jango Fett who became a member of the True Mandalorians and one of Mereel's lieutnants.
However once day Mereel was victim of a trap set by Vizzla with the complicity of one of Mereel's own right hand men called Montross, and mortally wounded, dying in his adopted son's arms. Jango would then expose Montross' betrayal and become the new Mandalore, leading the True Mandalorians for several years.
One day Jango and his men were hired by the governor of a planet called Gallidran in order to put down an insurrection. Unknown to them the governor was in league with Tor Vizzla who had set again a trap for the True Mandalorians, having framed them for the massacre of pacifical activists and of many innocent civilians who had been in fact murdered by the Death Watch. The governor then asked the Jedi Order and the Republic for help, to which the council answered by sending a strong Jedi force led by none other than Dooku who was one of the most powerful and respected masters of the order at the time.
Seeing that they had been set up Jango tried to flee with his forces but they were confronted by Dooku and the other Jedis and a battle ensued in which Jango's men were killed while taking many Jedis with them. Jango was taken prisonner by the Jedis and delivered to the corrupt governor who sold him to slavery. Dooku would later discover the truth about the governor and the crimes for which the Mandalorians were framed and became disgusted by how easely the order came to believe the governor and didn't investigate before confronting the true mandalorians. This would become one of the factors in his dissilusionement with the Jedi council and the republic and his eventual fall to the dark side.
As for Jango he spent several years as a slave until he managed to break free, retrieve his armor and find back Vizzla that he ambushed in Corellia's space. The chase between the two would come to Corellia itself where a brutal fight happened where Jango gained the upper hand on Vizzla until the Death Watch leader injected him with a poison to weaken him. Ironically this action would save Jango as two ferocious dire-cats attacked Vizzla and devoured him but sparred Jango as they were repulsed by the poison inside his body.
This is after all these events that Jango would become a bounty hunter and would eventually meet back Dooku who would recruit him to become the source of the clone army on Kamino.