This new Search mode lets you find almost anything within the Installed Apps. If you are still Wondering "How This feature works", Google made it easier to understand with few examples-
- Find your contacts and messages. Easily find the friend you want to catch up with, or the name of the new sushi place that your friend told you about last month — just search for [sushi] and find the message.
- Listen to your favorite running song or watch that sneezing panda video for the 15th time — all in one place.
- Stay organized with your tasks and notes. Want to check off items on your grocery list? No problem, just search for [groceries].
For now, This "In Apps Search" feature works only for few Apps including Gmail, Spotify and YouTube.
To access this feature, go to the Google app on your Android phone and find the In Apps tab. This feature works without WiFi or Cellular Network
Source- Google Blog