An aspect ratio is the width-to-height ratio of an image.
It is typically expressed as two numbers separated by a colon, such as 7:4 or 4:3.
A square image has equal width and height, described as a 1:1 aspect ratio.
The image could be 1000px × 1000px, or 1500px × 1500px, and the aspect ratio would still be 1:1.
1920 x 1080 is a 16:9 aspect ratio.
By default, smartphones, DSLRs, and most modern camcorders record video at 1920 x 1080.
The 256:135 (1.90:1) aspect ratio is the FULL container aspect ratio of a Digital Cinema Initiatives (DCI) projector.
This means that it forms the largest frame a DCI projector can achieve.
It's for this reason that most high-quality consumer mirrorless cameras record at the 1.90:1 aspect ratio as standard.