The Flow Starter node allows you to start a Flow (Flow 1) from another Flow (Flow 2).
The triggered Flow will start immediately and not wait for the triggering Flow to be completed.
You can pass custom variables from the triggering Flow to the triggered Flow using the Start node. Keep in mind that when starting a Flow from the Flow Starter node some of the Start variables in triggered flow will be empty:
This node is very similar to the Flow Component node, the difference is that when a flow is started from the STARTER node a new Flow Run is started. When a Flow is started from the COMPONENT node there is no new run but the nodes of the used flow are added to the triggering flow(like the Nested UI in UIs).