"Your Google Account was just used to sign in from Chrome on Mac. Why are we sending this? We take security very seriously and we want to keep you in the loop on important actions in your account. We were unable to determine whether you have used this browser or device with your account before. This can happen when you sign in for the first time on a new computer, phone or browser, when you use your browser's incognito or private browsing mode or clear your cookies, or when somebody else is accessing your account."
Google suggests to check the Devices & activity section from Google Accounts Settings if you don't recognize this activity. You can also find additional information about the browsers and locations that were used. For mobile devices, you can even remove account access to make sure it can no longer access your Google Account.