Senly groups mail automatically by conversation. A thread is every back-and-forth message about the same subject between you and the other party — just like Gmail does.
In the thread list, you see a small number behind each subject (e.g. 3) when there are multiple messages in it. When you open the thread, only the latest message is expanded by default — all earlier messages are collapsed so you do not have to scroll through hundreds of lines.
Click a collapsed message to expand it. Or click Expand all in the top right to see the entire thread at once.