You must be a veteran
You must have service-connected disabilities
You must not have been discharged dishonorably
You must have at least one service-connected disability rated at least at 60%, OR two or more service-connected disabilities with at least one disability ratable at 40 percent or more with a combined rating of 70 percent or more.
You must be unable to maintain substantially gainful employment as a result of service-connected disabilities (marginal employment, such as odd jobs, is not considered substantial gainful employment for VA purposes).
TDIU stands for total disability based on individual unemployability. The VA uses the term “Individual Unemployability” but many people add-on the TD part to emphasize that you are getting full compensation as if you have a 100% rating.
Read more about TDIU benefits for veterans here: