TCP provides a reliable, end to end, bi directional byte stream service to the application using it.
In the byte stream it provides every byte a unique sequence number.
The sequence number field in the TCP header indicates the position in the byte stream of the first byte in the TCP data field.
Example: If the sequence number of the first byte in the byte stream is 1000 and this is the first segment, then the sequence number of this segment will be 1000.
If the segment is 500 bytes long then the sequence number of the next segment will be 1500 and so on.