Definition of Open-ended
Open-ended" means a print that can be touched from both sides, from above and below. If this is not yet perfectly clear to you, there is nothing better than an example. You receive 4-5 preflops. If the flop falls 6-7-x or 2-3-x, you get a draw at straight Open-ended. In the first case, a 3 will give you the big straight and an 8 will give you the small straight. In the second example, same principle, you can complete your small straight with the 5 or your big straight with the ace.
The importance of distinguishing between small and big straights
In our example above, we say that with 4-5 over 6-7-x, we would have a big straight if a 3 fell on the turn and a small straight if an 8 fell on the turn. The reason being that the big straight will give us the best possible straight. On 6-7-x-3, the best straight is made with 4-5. But if you have 4-5 on 6-7-x-8, your straight would be beaten by 5-9 or 9-10. Draw Open-ended increases our chances of completing it. With 4-5 on 2-6-J you have 4 cards that can give you a straight (the four 3's for a non-Open-ended draw) while if the draw is Open-ended as on 5-6-x, you will be helped by 8 cards!