t is the value of the variable t, which is set to 0 initially
Then when someone presses a key while that input is focused it will look at the value of t, which is 0 the first time, which will be interpreted as false, so s will be set, and then t is set to 1. Then if they press another key, t will be one, and e will be set.
So, first time s will be set, any time after that e will be set.
Looks like the script attempts to display the number of key strokes per second (but fails because it doesn't take into account that people can remove text and enter new test instead, which should also amount to the number of key strokes I'd say)