“ready” is a concept in Godot which is a part of the Node Lifecycle. When a node is ready it will call the _ready() method . After that method is called, then the ready signal is called.
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_ready() is called from bottom to top
Once all nodes have been added in the scene tree, they receive the NOTIFICATION_READY notification and their respective _ready callbacks are triggered. For groups of nodes, the _ready callback is called in reverse order, starting with the children and moving up to the parent nodes.
This means that when adding a node to the scene tree, the following order will be used for the callbacks: _enter_tree of the parent, _enter_tree of the children, _ready of the children and finally _ready of the parent (recursively for the entire scene tree). - From docs