Summarize this article in one sentence.
This will reveal the Search bar.  You can search for messages that were sent or contacts that you've chatted with. WhatsApp will search all of your conversations to find matches. This will open the conversation and highlight your search term in it.

Summary:
Tap the WhatsApp app on your Home screen. Tap Chats. Swipe down. Tap the Search bar. Type a search term. Tap a conversation from the search results.