What does a left renal agenesis mean?

Absent left kidney: Means that the left kidney failed to develop, so there is only a right kidney present. Most people can lead a perfectly normal life with one kidney. They should avoid excess protein in their diet and minimize salt intake (good advice for everyone).

Renal agenesis: It simply means you were born without a left kidney. People go through their entire life without a kidney or donate a kidney. The amazing thing about kidneys is we are born with more than we absolutely need and having only one is not a serious problem unless, you damage that one and then there might be problems.