Source of the picture: comics.com
As exemplified by the comic strip above, in all languages the reference of certain words and expressions relies entirely on the situational context of the utterance, thus can only be understood in light of these circumstances. This aspect of pragmatics is called deixis (from the Greek adjective deiktikos, meaning 'pointing, indicating'). We could also say that deixis is the process of 'pointing' via language. The linguistic forms we use to accomplish this 'pointing' is called deictic expression.
There are some sentences in English (like the one above) that are virtually impossible to understand if we don't know who is speaking, about whom, where and when. Deictic expressions are among the first forms to be acquired and spoken by very young children. They can be used to point to a person via person deixis (me, you), to a location via spatial deixis (here, there) or to time via temporal deixis (now, then).
Are you already capable of identifying different types of deixis? Then test yourself, click on the italicized sentence above and name the various kinds of deixis.
If you are not yet familiar with this topic, click on the terms below, read through the information given there and test yourself later.