I've got this sentence in an email from an internet site where I learn foreign languages. What does it mean?
Way to go! You are on a 1 day streak!
A streak is
a period of repeated success or failure
In this case, it's a task you have fulfilled a consecutive number of days. A 1 day streak isn't really a streak, because there is no 'repeat'. If you fulfil the task tomorrow again, you will be on a 2 day streak. Next day, you could be on a 3 day streak. If at any day in the future you don't fulfil the task, you will be counting from 1 again.