What were the "Bigdei S'rad"?
Refer to 31:10:1:1, 2.
Why does the Torah add "Ka'asher Tzivah Hashem Es Moshe" here and by the termination of each subsequent Melachah that the workers performed?
Seforno (on Pasuk 5): To teach us that, by each and every Melachah that they performed, they had in mind to carry out the will of Hashem as He commanded Moshe.
Ayeles ha'Shachar: It is hard to fulfill exactly without any deviation. The Torah repeats "Ka'asher Tzivah..." each time, to teach that they did exactly, without deviation.