1t1d stands for "One Thing One Day". It is a recurring project held by Transtagonist.
For each project, the creator makes one "thing" of a specific type for each day of a predetermined period of time. A "thing" can be anything: a drawing, a short film, a birdhouse, a story, etc. The period of time is also open-ended, though two weeks is a typical time.
Other special rules can be applied to a project. For example, before starting, a phrase is often assigned to each day for inspiration.
An artist or craftsman often needs guidance, challenge, and inspiration. Working within limitations can be both restrictive and helpful. Unusual limitations like short production time make a creator need to work faster, and often make him|r think in a different way.
That said, 1t1d ends up being about whatever the creator makes it, but there is usually a focus on quantity of output, as well as routine.