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.