These are almost certainly parasites called Scale insects which suck the juices out of the stem and leaves and cause the plant to lose vigor. In this case it is a Coccus. Fortunately, you have spotted them early, before your avocado has become infested with them, and they can easily be removed in one of the following ways:
Scrape them off with a cotton swab that you have dipped in rubbing alcohol;
Spray them with a home-made or insecticidal soap solution once every two or three weeks (see here); you will need to do this at least three times to prevent them from returning.
There is a useful article about these bugs and how to control them, here, and another, very detailed one, by the Utah State University Extension, here.