"Customizable widgets" refers to small applications or tools on a device or website that can be adjusted or modified according to individual preferences. Full definition
To the left you'll find your notifications as they come in, and to the right you'll find a succession of customizable widgets. (androidcentral.com)
Going to the right, you'll find various customizable widgets. (9to5google.com)
Many of these watch faces support customizable widgets that display useful information, such as current weather, daily step counts, and missed call notifications. (androidcentral.com)