What is the meaning of "va'Yasi'u"?
Rashi, Targum Yonasan: They uprooted [from the mountain]. Radak - these are the big rocks used for the foundation - "rocks of ten Amos and rocks of eight Amos" (7:10).
What are "Yekaros"?
Rashi: They are heavy. Radak - even though it already said Gedolos, this teaches that they were heavy to move 1 .
Targum Yonasan: Yeakrin. Radak - there was a known mountain, that its rocks were better than those quarried from elsewhere, therefore they were Yekaros, for they were found only there. Anything not available is called Yakar.
Malbim: They were high quality, i.e. pure marble. Even though normally people put inferior rocks for the foundation, Shlomo commanded to use big, quality rocks, and they should be Gazis.
Perhaps he means that they were hard to move. I.e. they were dense, or they were rough at the bottom and move (slide) only if a great force is applied. (PF)
What are "Avnei Gazis"?
Rashi (from Sotah 48b): They are hewn. Even though it says (6:7) "chisels, all iron tools and a saw were not heard in the Bayis when it was being built", they cut the rocks outside, and brought them in to build with them. Malbim - these were used for the foundation 1 .
Radak: They are the other, small rocks, for above, and not for the foundation. It says (7:9) "Avanim Yekaros k'Midos Gazis"; they had a known measure. We find that the measure of Avnei Gazis 2 was five Tefachim (Bava Basra 1:1); Shlomo's rocks were bigger. It should say here v'Avnei Gazis; the prefix Vov is omitted, like in "Shemesh Yare'ach" (Chabakuk 3:11).
That is the thickness of a wall of Avnei Gazis! Perhaps Radak infers that the rocks themselves were five Tefachim thick, for it says there that a wall of uncut rocks is six Tefachim wide. If five Tefachim were for two or more layers of cut stone, we would not need to add a whole Tefach for uncut rocks. (PF)