What are "Moseros"?
Rashi: They are the straps of a yoke (to tie it on the animal's neck).
Radak citing Targum Yonasan: They are Chankin (chains or ropes around the neck).
What are "Motos"?
Radak (here and 28:13): They are straps and sinews 1 used to tie a yoke on the animal's neck.
Radak citing Targum Yonasan: They are sticks.
When did Yirmeyah put the Moseros and Motos on in his neck?
Rashi (3): It was in Yehoyakim's first year. They were on his neck for 15 years, until Chananyah broke them in the fourth year of Tzidkiyah's reign (28:10).
Radak (28:1): It was in the first year of Tzidkiyah's reign. He put one on his neck [and sent the rest to the kings].
Why did Hashem tell him to make the Moseros and Motos?
Radak (1): It is a sign that the kings should submit to the yoke of Nebuchadnetzar. Chazal said that when Hashem made Nebuchadnetzar king, he gave him authority over the kings listed (in verse 3).