2) What is a Phrase?

Not all word groups make complete sense.
 

In that school,       On the table,         During his weekend,

Each of those word groups makes some sense.  None of them can stand alone. Word groups such as those are phrases.

Sentence / Phrase

It is important to understand the difference between a sentence and a phrase.

The old man sat down.              He switched on the radio.

After dark     At last        For nearly an hour