Metafields help you customize the functionality and appearance of your Shopify store by allowing you to save specialized information that isn't captured by default in the Shopify admin. Metafields can be used internally in your Shopify admin and you can also choose to display them in your online store. With Rapid Search's new feature, your customers can search in Products' metafields information.
Metafields can be added to your store in your Shopify Admin > Settings > Custom Data menu. Here, you can find all kinds of options, but in our case, the Product metafields will be relevant. Currently, Rapid Search can search in the following 4 types of metafields: Single-line text (one value and list of values), multi-line text, and rich text). 
After creating your Product metafields, you can go ahead and assign the metafields to your products. You can add metafields to each product separately in their product menu, but for a time-saving solution, we recommend the Bulk edit option. In your Shopify Admin > Products menu, select multiple products and click the Bulk edit button. 
On the next screen, we recommend choosing to edit your visible Columns to Product Title and Metafields for simplicity. You can then click into the first row of each product to populate it with the selected Metafield option.
Optional extra step: If you'd like to display the metafield information on your product pages, you need to set up a designed place to display them. You can do so in your Shopify Admin > Online Store > Theme > Customize menu. Please select which area you'd like to customize by selecting the Product at the top of the page. 
Then, within the Template section, please select "Add block" with your desired settings. 
Please populate the new block's content with a dynamic source like so: 
For example, a candle seller might want to display the burn time for candles on their product pages, whereas a car parts store might want to display product compatibility or substitutes for other mechanic parts.
Take an example of a store that sells clothing and apparel, and one metafield is the availability. The store offers two options:
A) an item is ready to ship, as it is already manufactured, or,
B) the item is made-to-order, so the customer can expect a longer processing time but has the option to adjust some details in the size.
Let's say the customer is looking for a skirt for a competition: if the competition is in 2 weeks, they will want to have the skirt ready ASAP, whereas if it is in 2 months, they might prefer having the size adjusted. Entering the keywords "ready to ship" or "made to order" will help the customer find the best options for them.
Searching in metafields is now possible with Rapid Search, supporting the search in 4 types of metafields: Single-line text (one value and list of values), multi-line text, and rich text).
This feature is disabled by default, but you can enable it by going to the Rapid Search Admin, Settings, Advanced menu, and setting a "search field weight" for the metafields.
This article focuses on the option to search in metafields with the Rapid Search app, but if you'd like to learn more about Metafields we recommend checking out this YouTube video and Shopify Article.
Please note, that filters based on metafields is a separate functionality that is currently not yet available with Rapid Search. If you are interested in this option, kindly visit the Rapid Search Admin, Feature request tab, and make sure to upvote and Subscribe to the "Metafield filter" feature request so you can get notified when it is added to the app.