Like a lot of text messaging clients, WhatsApp shows when a recipient views a text. That can be annoying and intrusive. If you want to disable those last seen messages, there's an easy way to do it.
On iOS and Android, head into the app and tap Settings > Account > Privacy > Last Seen, then change it to "Nobody". You can also use a third-party app plugin such as Last Not Seen on Android to hide that timestamp so people don't know when you're reading their messages.