Priming is discussed in the context of the activation theory. It is assumed that concepts that have some relation to each other are connected in some mental network, so that if one concept is activated, then concepts related to it are also activated.
Priming is a phenomenon related to this concept. It can be shown in the following example:
A subject is shown the word nurse. Presumably the subject will then think of other words related to the word nurse. If the subject is then shown either the word doctor or the word butter, the subject should be able to read the former word more quickly than the latter word because doctor is related to nurse and therefore has been recently accessed, and so more familiar to the subject.
The word nurse then serves to "prime" the second word, doctor.