A pun used in “Sonnet 1” is a play on the word tender, meaning both young and money.
A pun is a play on words. It often involves double meaning. In this poem, we have a young man being encouraged to go forth and procreate. He is in the spring of his youth, and has no children. The speaker is encouraging him to
And only herald to the gaudy spring,
Within thine own bud buriest thy content,
And tender churl mak'st waste in niggarding
In this case, the “tender churl” is ironic, referring both to the idea that the young man is ripe and ready to start a family, and he should not spend his youth and beauty (tender, as in money), or waste it, because it will not stay long. By not marrying and having children, he is wasting himself.