Shared ground
Paul’s main point is that worship speech should be understandable to others when it is meant to serve the gathered group. In this paragraph he treats “tongues” as a kind of speech that, without interpretation, does not communicate meaning to listeners. So he says tongue-speakers should “pray that [they] may interpret” (v.13). He contrasts spiritual activity (“my spirit prays”) with the mind’s grasp of meaning (“my understanding is unfruitful,” v.14), and he prefers prayer and singing that include both (v.15).
Paul also ties understanding to shared participation. If a person blesses or gives thanks “with the spirit” but others cannot follow the content, the “uninformed” person cannot honestly say “Amen” (vv.16–17). Paul is not rejecting tongues outright; he says he speaks in tongues extensively (v.18). Still, “in the assembly” he values a few intelligible words that instruct others over many unintelligible words (v.19). Compare 1 Corinthians 14:1.
Where interpretation differs
Some disagreement centers on what kind of “tongue” Paul means. One reading says it is a real human language that others in the room do not know; “interpret” would then mean translating into the common language. Another reading says it is a Spirit-prompted form of speech that is not a normal human language; “interpret” would then mean explaining the message so it becomes intelligible.
Another difference concerns “spirit” and “understanding” in v.14–15. Some take “spirit” mainly as the speaker’s own inner life engaged in prayer while the mind does not grasp what is being said. Others take “spirit” as especially highlighting the Holy Spirit’s activity, while still agreeing Paul expects the mind/meaning to be engaged for the group’s benefit.
Why the disagreement exists
Paul does not define “tongue” in this paragraph, and the word can refer either to languages or to unusual speech. Likewise, “interpret” could fit either translation (if it is a known language) or explanation (if it is not). The “spirit vs. understanding” contrast is clear in function (private spiritual activity vs. graspable meaning), but less explicit about whether “spirit” here points mainly to the speaker’s spirit or the Holy Spirit.
What this passage clearly contributes
Explicit in the text: Tongue-speakers should seek interpretation (v.13). Prayer and singing are best when “spirit” and “understanding” are both involved (v.15). In group worship, the ability of others to understand matters for their participation (“Amen”) and strengthening (vv.16–17). Paul sets a strong priority for intelligibility in the gathered meeting (v.19).
Reasonable theological inference: Corporate worship is not only vertical (speaker-to-God) but also horizontal (building others up). Meaningful participation by ordinary attendees is treated as a real measure of whether the gathering is being strengthened, not merely the speaker’s sincerity or intensity. See also 1 Corinthians 14:26.