GH: Yibum is when there are two (or more) paternal brothers, one of whom dies childless. The brother marries (or does chalitzah if he does not want to marry) the widow. This marriage is called yibum (yevama reverse to the widow; the plural is yevamos, but the actual name of the mesechta (tractate) is likely actually “Yabmus” which refers to the topic of this type of marriage).
If two brothers are married to two sisters, and one brother dies, he cannot marry his sister in law, because she is his wife’s sister, and a man cannot marry two sisters.
This case, with several variations, is frequently discussed in the Gemara.