Well, the story starts even before the WF subscription and association comes into picture. When we use CSOM, it is important to understand that how a workflow is invoked programmatically. There exist a Workflow service Manager which is responsible for getting all workflows for a site. In MSDN words, "This Describes the workflow host configuration states and provides service objects to interact with workflow instances."
Now lets talk about WF Subscription service and WF association.In simple words:
Workflow subscription service: It's one instance manages All workflow associations.
Workflow Association: It simply describes the association of a WF
to a particular SharePoint component (e.g association with List or
Needless to say that you will require WF subscription to get association. Please refer the sample code:
var workflowServicesManager = new WorkflowServicesManager(ctx, ctx.Web);
var workflowSubscriptionService = workflowServicesManager.GetWorkflowSubscriptionService();
var workflowAssociations = workflowSubscriptionService.EnumerateSubscriptionsByDefinition("guid-of-workflow");