The ESP32 development board packs all the essential components, including power circuitry, programming interfaces, LEDs, and crucially, an onboard PCB antenna for Wi-Fi/Bluetooth connectivity.
 
The inclusion of a PCB antenna is supposed to ensure wireless communication right out of the box. The onboard antenna is usually some variation of a printed inverted F-antenna (IFA for short). This type of antenna is sensitive to the ground plane size, meaning that DevKit cannot be too small to ensure optimal performance.

An IFA is also quite fussy about its placement. DevKits follow a simple rule: antennas are placed on the edge of the board far away from connectors and big metal objects.

Another important note - any additional dielectric (like enclosure, potting, human hand) has a large potential to detune the antenna or reduce its performance. An onboard antenna is fine-tuned to work "as it is", so any unexpected additions will have an effect on wireless connectivity.
 
 Luckily, some variants of ESP32 development boards offer the flexibility of connecting an external antenna, removing all the limitations of PCB antennas.