What Did Einstein Mean when He Said Space is Curved?

[Anomalocaris]

How does he define the word curved as he used it to describe space?

He meant well known geometric principles known since Euclidean to be true for angles and lines inscribed on a flat plane would not hold true with physical representations of these lines and angles created on sufficiently large scale in space near sufficiently massivee objects.  Instead these physical lines and angles would behave as if they were inscribed on a curved surface.