What does this word mean in physics language?

They mean the same as they do in English.

 

You said it in your post. For instance, velocity can be considered relative because it needs a reference point - generally, it means that you need 2 pieces of information for a quantity to be meaningful. So if I say somethings has a speed of 5m/s, that's meaningless, but if I say 5m/s relative to that lampost, then it has meaning.

Arbitrary - exactly that. When comparing quantities relative to each other and a third reference point, we can use arbitrary units. E.g. if one car in a drag race is moving 2x faster than another relative to the start point and we use arbitrary time units, it does not matter what units we use (metres, feet, planck lengths, etc), the relative velocity will be 2x. In general, that means no specific reason for choosing a unit/point of reference.

 

Arbitrary point/quantity value x: any point or quantity value from the space of allowed positions/values.

The expression " relative to B" can have different meanings:
sometimes it means "in coordinate system fixed to B", else it can mean "when the value at B is subtracted".
Those two meanings are sometimes identical, sometimes not. For example: speed of object A in the system fixed on object B is
v(A)-v(B) in classical physics, but not in the theory of relativity.