OpenVPN is a widely-known user of TUN/TAP.
TUN/TAP provides packet reception and transmission for userspace programs. It can be seen as a simple Point-to-Point or Ethernet device, which, instead of receiving packets from physical media, receives them from a userspace program and, instead of sending packets via physical media, writes them to the userspace program. https://www.kernel.org/doc/html/latest/networking/tuntap.html
TUN/TAP acts as a virtual Network Interface Card (NIC). Compared to a real NIC, instead of getting packets from the hardware, TUN/TAP gets them from a userspace app. Then, these packets are parsed by the kernel. In the same way, as if they had been received from the hardware.
When OpenVPN is in use, the packets sent by userspace apps 1 are routed through a virtual TUN/TAP-managed interface 2. These packets are then delivered to the OpenVPN app 3, which adds the necessary encryption and sends them out through a hardware-backed interface 4 5 6.
A reverse process happens for packets received back from the hardware.