Conventions vary by field, but within a specific field they're usually pretty consistent. In natural sciences (except large physics papers) the convention is that the first author is the one doing most of the practical work. The last author is the PI (principal investigator) of the group who had a hand in designing the experiments and oversaw the research. Now, the latter can mean anything from barely doing any work on the paper to being deeply involved in the research.

If you're reading papers most of the time the last author is more meaningful to you because they're the senior researcher, you know their research interests and what kind of papers they produce. The first authors are PhD students and PostDocs, they change much more often.