The poster presentation is a fundamental element of training in biomedical research. It demonstrates several important competencies: research productivity, the ability to present and display experimental data in a coherent fashion, and the ability to orally present and answer in-depth questions about the material to those who view the poster. It is less meaningful than an authorship but is still worthy of inclusion on an app.
The most important poster sessions are at national conferences: Neuroscience, Experimental Biology, many others that I do not list. University-wide poster sessions are still worth mentioning if you have no other evidence of your research productivity, but don't expect it to move mountains and open doors at Stanford.