Dual SIM means that there are two SIM slots within the device - a physical SIM card and an eSIM. Like a traditional SIM card, the eSIM stores data that's needed for your device to use the EE network.
Unlike a SIM card, an eSIM is downloaded onto your device.
With the Google Pixel 3 and Pixel 3 XL only one SIM will work and be able to receive a single signal at once, so you'll have to manually choose which SIM is active within your phone settings.
The Google Pixel 3a, Pixel 3a XL, Pixel 4 and Pixel 4 XL are Dual SIM Dual Standby if the latest software is used, this means both SIM lines are active.