Something or someone respectable is honest, good, and proper. Respectable behavior includes things like contributing to charity, volunteering at an animal shelter, and helping your friends study vocabulary.
Anything or anyone respectable deserves respect for being honorable or moral. If you return a wallet that you find on the street, that’s respectable behavior. People would probably view you as a respectable person. However, if you removed all the money from the wallet before returning it, that’s not respectable. Respectable can also be used to describe to an adequate or large amount of something. If you return the wallet untouched, you might get a respectable sum as a reward.