TCP is a transport protocol that provides an ordered, reliable byte stream between applications. It handles details such as retransmission when information is lost. Reliability here describes delivery behavior, not whether the content or the person at the other end is trustworthy.
A practical example
A file transfer needs the complete file even when some traffic is delayed. TCP can retransmit missing information, which helps deliver the stream but can add waiting. The result you observe may be a pause rather than a visibly damaged file.
What to check
If an app gives you transport choices, first use the documented default. Change one option at a time and record whether the task completes, stalls or produces an error. A failed connection does not identify a protocol problem on its own. Keep network type and test time constant when making a comparison.
What it does not tell you
TCP does not itself mean the connection is encrypted. Encryption can be supplied by another layer, such as TLS. Do not choose a VPN based only on whether an advertisement mentions TCP; ask which protocols and settings its actual app supports.
Read the technical reference for the underlying definition. For a connection that is failing now, use the troubleshooting guide. To choose an app, follow the VPN selection worksheet.