Arduino IoT Cloud is a service that tries to make it seamless to convert your Arduino devices into IoT devices.
Any IoT device typically gathers data from sensors, does some processing onboard, and transmits either the raw or the processed data to a server. Arduino IoT Cloud allows you to generate a digital twin of your device (called as a thing), add variables and settings to that digital twin, and then generates the Arduino Sketch automatically which you can upload to the device. Thus, you essentially don’t need to write the Arduino Sketch yourself.
What’s more, it also provides dashboards and widgets to monitor your devices, and also tracks events (i.e. special application messages sent by devices, indicating that something has happened) as and when they are received.
It is important to note that not all Arduino devices are compatible with Arduino IoT cloud.Only a select few with WiFi or GSM/NB-IoT provisions are compatible. Some third-party boards are also compatible.
You can get a list of all the compatible devices, and know about other salient features of Arduino IoT Cloud here.
Also refer to this guide to get started with Arduino IoT Cloud.