Clientless remote access is when network access is granted to a device in a remote network without the need to install a specialized program(s) on the connecting device.
An example would be SSL VPN whereby since most browsers support SSL, they can enable the use of SSL VPN for secure connections to remote networks and resources. IPsec VPN, on the other hand, requires specific client-side software in order to establish a VPN connection to an IPsec VPN server in a remote network.