1. On: This is the most common meaning, and it's often used in the same way as "on" in English.
* Ar y bwrdd: On the table
* Ar y ffordd: On the road
* Ar y glaw: In the rain (literally "on the rain")
2. At: This meaning is similar to "at" in English, but it's not used as often.
* Ar y siop: At the shop
* Ar y dŷ: At the house
3. To: This meaning is used in certain contexts, especially with verbs like "go" or "come".
* Mynd ar y ffordd: To go on the road (to go away)
* Dod ar y ffordd: To come on the road (to arrive)
4. In: This meaning is less common but can be used in some contexts.
* Ar y gors: In the bog
5. By: This meaning is sometimes used in conjunction with verbs like "sit" or "stand".
* Eistedd ar y gadair: Sit by the chair (sit on the chair)
The best way to understand the meaning of "ar" is to consider the context in which it's used.