Hishstadlus What is it?

Having spent many hours on the texts regarding parnossa. I have been amazed at the lack of clarity regarding  hishstadlus. It is rarely mentioned in primary sources such as Tanach or Talmud. It is mentioned with regard to another unclear concept bitachon. Yosef is punished for too much hishstadlus. The most direct discussion I have found is in Mesilas Yeshorim 21. I also could not find it in Rambam

Mesilas Yeshorim (21:07) However, that which can protect a person and save him from these detriments is trust in G-d. Namely, that a person casts his burden entirely upon G-d, knowing that it is certainly impossible for a man to lack what was designated for him, as our sages taught: "all of a person's sustenance [for the year] is fixed for him from Rosh Hashana [to Yom Kippur]" (Beitzah 16a). Likewise, they said: "no man can touch what was prepared for his fellow even to the extent of a hair's breadth" (Yomah 38b).

Mesilas Yeshorim (21:08) A person could have sat idle and the decree would have been fulfilled (his designated portion would have come to him), had it not been preceded by the fine imposed on every human being: "by the sweat of your brow shall you eat bread" (Gen.3:19), whereby a person is required to make some effort for obtaining his livelihood, for thus the exalted King decreed. 

Mesilas Yeshorim (21:09) This is like a tax imposed on the human race which one cannot escape from paying. Therefore, our sages, of blessed memory, said (Sifri 15:18): "I might think one can sit idle, but scripture says (Devarim 28:20): 'in all that you set your hand to do' ".

Mesilas Yeshorim (21:10) Only that it is not the efforts (hishtadlut) that help. Rather, the efforts are necessary, but once one has put in some effort, he has already discharged his obligation and there is place for the blessing of Heaven to rest upon him, and he need not consume his days in exertion and labor. This is what king David said: "For not from the east or from the west, nor from... but it is G-d who executes judgment, [putting down one and lifting up another]" (Tehilim 75:7-8), and king Shlomo said: "Do not weary yourself to grow rich; cease applying your understanding" (Mishlei 23:4).

Mesilas Yeshorim (21:11) Rather, the true path is that of the "early Pious ones", who made their Torah primary and their work secondary, and succeeded in both (Berachot 35b). For once a man does a little work, from then on, he need only trust in his Master, and not be distressed by any worldly matters. Then his mind will be free and his heart ready for true Chasidut and perfect divine service.

Duties of the Heart (Bitchon 3:36)  Secondly, if a man were not forced to exert himself in seeking a livelihood, he would kick (become defiant) and chase after sin, and he would ignore his debt of gratitude to G-d for His goodness to him. As written: "And the harp, and the lyre, the timbrel, and flute, and wine, are in their feasts: but they regard not the work of the L-ord, neither consider the work of His hands" (Yeshaya 5:12), and "But Yeshurun grew fat, and kicked: you are grown fat, you are grown thick, you are covered with fatness; then he forsook G-d who made him, and lightly esteemed the Rock of his salvation" (Devarim 32:15). And the sages said "it is good the study of torah with working for a livelihood because the toil in both removes thoughts of sin, and all torah study without work will in the end be abandoned and bring to sin" (Avot 2:2). And all the more so for one who has no share in either torah or work, nor directs his attention to any of these pursuits.