What is the meaning of the word "Keidmah"?
Rashi: It means 'on the east side' as Degel Machaneh Yehudah was the first to travel. 1
Rashi: Which is synonymous with 'in front' or 'first'; and the west is therefore called "Achor" ('at the back' or 'last').
Why did specifically Yisachar and Zevulun join Yehudah to form Degel Machaneh Yehudah?
Ramban (on Pasuk 2): Together with the three tribes of Degel Machaneh Re'uven - Reuven, Shimon and Gad, they comprised the sons of Le'ah - including Gad, 1 the firstborn of her maidservant Zilpah.
Why did the three tribes that comprised Degel Machaneh Yehudah (Yehudah, Yisachar an Zevulun) encamp specifically in the east?
Ramban #2 (citing Midrash Rabah) 1 : Hashem said to Moshe: 'Yehudah, the designated sovereign, 2 shall encamp in the east, from where light comes to the world; next to him, is Yisachar, the master of Torah, 3 and then Zevulun, the master of wealth. 4 They will travel first!' 5
Which is called 'light' (See R. Chavel's footnotes).
See R. Chavel's footnotes.