Genesis 4:25

And Adam knew his wife again, at the end of a hundred and thirty years after Habel had been slain; and she bare a son, and called his name Sheth; for she said, The Lord hath given me another son instead of Habel whom Kain slew.

וידע אדם עוד את אשתו, this occurred 130 years later than the last time, as is evident from 5,3. During the intervening years Adam had not sired any children because Chavah had been the cause of his being punished. He had kept his distance from her during all these years.

כי שת לי אלוקים זרע אחר, seeing that this time she had given the credit to G’d, saying שת לי אלקים, “G’d gave me,” she named the son שת.

כי הרגו קין, seeing that Kayin had killed Hevel she had not had any other seed from him. She perceived the birth of Sheth as G’d’s gift to her, compensating her for the loss of Hevel, who had not even left behind any offspring. This is why the Torah immediately continues telling us that Sheth, as opposed to Hevel, had offspring born to him whom he called Enosh. The meaning of the word גם, “also,” here is that “just as Chavah bore a son, so her son also sired a son, not only that but such offspring endured.” The Torah, significantly, does not tell us about Kayin’s offspring other than what we know about Lemech. The reason is that all of Kayin’s offspring was wiped out during the deluge. Noach was descended through Sheth.