‘ Then Jephthah fled from his brothers, and dwelt in the land of Tob, and there were gathered adventurers to Jephthah, and went out with him.'
So Jephthah had to leave his home and make his living as best he could in an unfriendly world. He had every disadvantage. He went to live in the region of Tob. Tob was an Aramaean city and area north of Gilead (compare 2 Samuel 10:6), possibly al-Taiyiba. It was named tby in the list of Thutmose III. But there his worth was recognised by similar stateless and rejected men and other adventurers who joined him under his leadership.
“Went out with him” indicates their purpose. They sought booty and spoils, probably attacking caravans, rustling and even attacking small towns and villages. And so, like David would after him, he developed skills in leading men, in fighting and in generalship, ready for when he would hear the call of Yahweh. He also built up a force of efficient, trained fighting men. It is probable also, that, like David, he did not attack his own countrymen, even possibly coming sometimes to their defence, otherwise they would not have considered him for the leadership.
We must beware of depicting him as too ‘rough'. He had grown up in an aristocratic household as a son of the house, and was used to good living. He had also had opportunity to develop his faith, even though he would have been excluded from much on the grounds that he was a bastard, although that may not have been generally known.