An asset group is a set of creatives that will be used to create an ad depending on the channel it’s being served on. It's a best practice to organise asset groups like you organise your ad groups, by a common theme. For example, you might try separating asset groups by the different products or service types you offer.
You can create one asset group when you build your campaign and add additional asset groups to already created campaigns. Google will automatically assemble the assets in an asset group into all applicable ad formats for your goal and show the most relevant creative for your campaign. The more assets you provide, the more ad formats the system can create and the more inventory your ad can appear on.
Learn more about how asset groups work.