The English noun tense comes from Old French tens “time” (spelled temps in modern French through deliberate archaisation), from Latin tempus “time”. It is not related to the adjective tense, which comes from Latin tensus, the perfect passive participle of tendere “stretch”.
In modern linguistic theory, tense is understood as a category that expresses (grammaticalizes) time reference; namely one which, using grammatical means, places a state or action in time.
The main tenses are past, present, and future.
Past tense signifies a time, when some state or action, has already taken place.
Last year, I read the complete works of Shakespeare.
Present tense signifies a time, when some state or action, is taking place now.
I read one book every day.
Future tense signifies a time, when some state or action, will take place.
I will read as much as possible in the coming days.
1. tempus. Charlton T. Lewis and Charles Short. A Latin Dictionary on Perseus Project. http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0059:entry=tempus
2. Harper, Douglas. “tense”. Online Etymology Dictionary. http://www.etymonline.com/?term=tense