Chess terminology contains terms from many different languages, including English, Persian, German, French, and Italian. En Prise (pronounced 'on pree') is from the French, en, or 'in', and prise, 'grip', 'grasp' or 'hold'. It can be translated as 'within reach' or 'able to be taken', and is used to indicate that a chess piece (other than the king) is exposed to capture. A piece can be said to be en prise even if the attacking piece can be recaptured. A quick illustration:
+----+----+----+----+----+----+----+----+
| | q | | | | | k | |
+----+----+----+----+----+----+----+----+
| | | | | | | p | p |
+----+----+----+----+----+----+----+----+
| | | | | r | | | |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | N | | | |
+----+----+----+----+----+----+----+----+
| | Q | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | | P | P | P |
+----+----+----+----+----+----+----+----+
| | | | | | | K | |
+----+----+----+----+----+----+----+----+
In this position, with White to move, there are two pieces en prise - the Black queen on b8 (which can be captured by the White queen) and the Black rook on e6 (also attacked by the White queen). With Black to move, there are also two pieces en prise - the White queen on b3 and the White pawn on h2 (both attacked by the Black queen). Contrary to appearances, the White knight on e4 is not under attack from the rook on e6 - the rook is pinned to the Black king by the White queen, and therefore is not a threat.
One of the first rules of thumb given to beginners in chess is to make two basic checks before making any move: Can I be placed in check? and Are any of my pieces en prise? Leaving a piece en prise is one of the most basic chess errors, and beginners who pick up the habit of checking for it will improve quickly.