What does the Pasuk mean when it states that the planks must be "So'amim" below?
Rashi: It means that the two Yados at the base of each plank must be cut in such a way that the plank will fit plush on to the socket that housed them in such a way that all the planks formed a straight line.
And what does the Pasuk mean when it states that the planks must be "So'amim" above?
What function did the rings mentioned here serve?
Rashi and Ramban #1 (citing a Beraisa): Joining every two adjoining planks via a ring that fitted into two grooves that were cut into the top of the planks, one near the end of each plank, would ensure that the planks did not move, and consequently, that they all formed a straight line on top, just as they did at the bottom. 1
Ramban #2: It refers specifically to the two corner planks on the north-west and the north-east. 2
Seforno and Rashbam (on Pasuk 28): This refers to the rings on the outside of the planks 3 that housed the bolts that the Pasuk is about to discuss. And the Pasuk is informing us here that rings were fitted on to the northern and southern ends of the western row of planks, into which the northern and southern bolts fitted. 4
See Ramban's objections to this explanation. He partially refutes his objections, by explaining that the Torah mentions the rings as if we already knew about them, since that is how one normally fits planks together. And the same applies to the word "v'es Tab'oseihem" (in Pasuk 29 [See also Seforno there]) and "v'es Vaveihem" (in Pasuk 37).
In order to firmly hold together the western wall with the north and south walls.
See Pasuk 30.
Thereby holding the western wall firm against the northern and southern walls (Seforno).