A generalized question can only get a generalized answer.
The most significant accomplishment is validating that the product you have built is a fit with your target market. This is demonstrated primarily by engagement (the people who sign-up or who previously visited, continue to return) and secondarily by growth, ideally based on word-of-mouth or viral growth but effectively converting paid traffic is a great second prize.
Other significant accomplishments include:
Not running out of money
Recruiting and retaining great talent who believe in the founders' vision.
Your loved ones not thinking you're as crazy as they thought you were a year ago.
I'm happy to talk to you in a call to give you more specifics about what you want to set as your goals more specific to your startup.