What is ANaphora Resolution?

Anaphora Resolution == the problem of resolving what a pronoun, or a noun phrase refers to.

In the following example, 1) and 2) are utterances; and together, they form a discourse.
1) John helped Mary.
2) He was kind.

As human, readers and listeners can quickly and unconsciously work out that the pronoun "he" in utterance 2) refers to "John" in 1).  The underlying process of how this is done is yet unclear... especially when we encounter more complex sentences:
An example involving Noun phrases (Webber 93)

1a)  John traveled around France twice.
1b)  They were both wonderful.  ??

2a)  John took two trips around France.
2b)  They were both wonderful.

Consequently, anaphora resolution presents a challenge, and is an active area of research.