Full mesh HA configuration

Hello,

in the full mesh high availability example in the FortiOS handbook (https://docs.fortinet.com.../full-mesh-ha-example) there are 2 switches before and behind the FortiGates, i.e. Sw1 - Sw4. It says succinctly "Establish an 802.1Q (Dot1Q) or interswitch-link (ISL) connection between them". (the two switch pairs) What is meant here? One L2 broadcast domain? What is going on these two links?

An explanation or a link to a document which explains this would be highly appreciated.

Thanks!

Ueli