Synthesis, Assets, Bootstrapping & Deployment
Synthesis
cdk synthThis traverses the app tree and generates CFN YAML and any assets that are needed.
Assets
This is any files that go with the CDK, such as code or Images. AKA the artefact that the app needs to run. This is bundled into cdk.out file.
Bootstrapping
cdk bootstrapCreates an S3 bucket to store the Assets. This must be done for any CloudFormation template over 50kb (apparently this means baso every time).
Deploy

App is init into an App Tree.
Artefact's are uploaded into CDKToolkit.
CFN Deployment begins.