What does 1 Corinthians 7:6 mean?

This short verse refers to a concession on Paul's part. However, it's possible to interpret this either in reference to the statement just made in verse 5, or to the one about to be delivered in verse 7.

If Paul is referring to the prior verse as a concession, he would be making it clear that mutual abstinence is something married couples may agree to for a time, not something Paul is commanding. The "concession" aspect would be Paul allowing for sexuality and marital agreements despite his personal preference for celibacy.

More likely is that Paul is prefacing his remark in the next verse: that he, personally, would like every Christian to be unmarried and unattached as he is. This verse, then, would mean to clarify that the words which follow are merely Paul's preference and his perspective. God does not command anyone to be like Paul in this way.

In the following verses, Paul implies that he's single and unburdened by strong sexual desire. He considers this a gift from God for himself. Seeing it as a good thing, he wishes more people had such a gift, for reasons he will reveal in the following passage. However, he understands that other people have other gifts and that God allows and honors both marriage and sex within marriage.