Yeah, that's a mystifying one, because you won't see it anywhere else.
- WR = World Record
- AR = American Record (outside the USA, this means "Area Record"; properly it should be NR or National Record)
- MR = Meet Record (it's ambiguous if this means Olympic Trials record or US Championships record - this event, regardless of setting)
- HF = Hayward Field (elsewhere this would be a "facility record," "venue record," or "track record" - generally of little consequence unless the venue has hosted many high-level events)
- WL = World Leader, fastest time in the world so far this year, in this event
- AL = American leader, fastest time in this event by an American this year, relevant only because the field for the Olympic Trials is limited to US citizens
- OS is the Olympic Standard, the time athletes needed to reach to qualify for selection to the team - a top three finish wouldn't matter if an athlete hasn't beaten this time in the qualifying period.
"Area Records" can also be called "Continental" records, since the IAAF's "Areas" align roughly with the continents: Europe, Asia, Africa, South America, NACAC, Oceania. You see a lot of these at Olympic and World Championships. The use, in the USA, of "American Record" to indicate a national record means the "AR" abbreviation can be ambiguous.
There's also a concept formally known as "All-Comers Records" which is similar to national records. A national record is the fastest time by a citizen of a given state, regardless of where it's run, e.g. if Nick Willis sets a national record in Boston, it's still the New Zealand record despite being run in the USA. The all-comers record is the fastest time inside the borders of a given state, regardless of the nationality of the athlete. For example, most of the all-comers records in China were run by foreign athletes at the 2008 Olympics or 2015 World Championships. All-comers records are a non-issue at a national event like the Olympic Trials, in which all athletes share a single nationality, but can be hyped by meet promoters in other contexts (and sometimes there's a legitimate thrill to it: "Nobody has ever run this fast here!")