Search a conversation
Once a conversation is long, scrolling back to a detail can take very long. The search looks for keyword through the chat you have open, so you can jump straight to the messages that mentioned it.
On a computer
Search this chat is the magnifying glass at the top right of the conversation. Click it and a field opens in the header, with the placeholder Find in this chat.
Type the word or the phrase you remember, and all matches are highlighted. The arrows beside the field step to the previous match or the next one, and a counter such as 1 of 2 shows which match you are on. The match you are on has a stronger outline, that way you know exactly where you are.
The × at the end of the field closes the search. The highlights disappear, and you are back to reading the chat as usual.
On a phone
On a phone, the same magnifying glass sits at the top right. Because the screen is smaller, search takes the place of the input box at the bottom instead of opening in the header. There you can type, and use the arrows to move between matches. Tap × when you are done so you can send normal messages again.