The tense of a verb is a form that (usually) defines whether you are describing an action of the past, the present or the future.
The reason English does not "have" a future tense is that there is no future form of the verb itself that would describe the future (as it is in, e.g. French: I will go>>J'irai)
In English, in order to give information regarding the future, we mostly use the modal will, but not always.
I will go the market tomorrow
I'm going to the market tomorrow
The market opens at 9:00
I'm going to open the box (i.e. I will open the box)
All of the above examples describe the future.