Many companies (particularly tech companies like Atlassian) grant their employees "share options" as part of their compensation. A share option is the right to buy a share in the company at a "strike price" specified when the option is granted.
Typically these "vest" after 1-4 years so long as the employee stays with the company. Once they do vest, the employee can exercise them by paying the strike price - typically they'd do that if the shares are now more valuable. The amount they pay to exercise the option goes to the company and will show up in the $2.3 million quoted in the question.