What does Straight Leg mean?

A style of jeans that have a consistent leg width running down to the bottom.

Heddels explains Straight Leg

As the name implies, straight leg jeans are jeans with a silhouette running straight down to the bottom. They have a consistent leg width and tend to feel somewhat snugger in the thigh. While occasionally confused with skinny jeans, straight leg jeans differ in that they do not taper towards the bottom but have an even leg width throughout. Straight leg jeans offer a comfortable, classic fit and are a popular cut for jeans today.

Additional Resources

Here is an example of the straight leg style: