A tense is a form taken by a verb to show the time of an action. There are three main tenses:
Present tense: things that are true when the words are spoken or written; or are generally true; or for some languages will be true in the future
Past tense: things that were true before the words were spoken or written
Future tense: things that will or might be true after the words are spoken or written
The tense can be shown in the verb. For example, am, is, and are are always present tense, and was and were are always past tense. Or the tense can be shown by adding words to the verb. In English, the words that we add to the verb are "helping verbs" or "auxiliaries", like be, have, shall, will, and so on. So we get the continuous present with is taking, the future with will take, and so on.
(Source: "Kids Encyclopedia Facts"
Here's a video that explains what tenses are for Chinese English learners. The video is about 8 minutes long and it is in Putonghua.