Deixis

Deictic words are language features that refer to the who, where and when of language. Words such as "you, here, now!" (a phrase beloved of fierce school teachers) describe the speaker's position in space and time.

This is also described as

words whose "meanings change quickly depending on the time or space in which they are uttered" (Leu et al.)

There are three main types of deixis:

personal: pronouns such as I and you
spatial: words describing the speaker in space or in relation to other objects such as here and there, come and go
temporal: words describing the speaker in terms of time such as now, then, yesterday and verb tenses.

So deixis provides context in relation to the speaker.
If Bob says "I swam over here" we hear Bob referring to himself in the context of a given place and time and we assess the situation in relation to where he is, was and where we are.
The viewpoint must be understood in order to interpret the utterance.

Additional deictic types include:

discourse deixis, empathetic deixis, social deixis and technological deixis (a reference to the forms and purposes literacy takes as technology changes the nature of literacy in general)
See http://en.wikipedia.org/wiki/Deixis