At (preposition)

1. Expressing location or arrival in a particular place or position.
2. Expressing the time when an event takes place.
3. Expressing the means by which something is done or achieved.
4. Expressing a particular rate or ratio.
5. Used to indicate a point in a scale or series.

Origin:

Old english æt, from an indo-european root shared by latin ad 'to, at'.

Examples:

1. We met at the restaurant for dinner.
2. The party will start at 8 PM.
3. She was surprised at how well he played the guitar.
4. The exchange rate is currently at 1.25 dollars to the euro.
5. He's an expert at playing chess.