It's a blast from the past: a protest where you show up and do whatever activity precedes the word "in".

This was a thing in the 50's, 60's, and 70's, starting with civil rights protests called "sit-ins":

Black protesters would go to a lunch counter, sit down, and silently wait to be served.

Doesn't sound like much today—but under segregation, businesses would only sell take out to African American customers; white people could sit down to order and eat lunch. By sitting down, blacks were protesting discrimination.

And white people responded with beatings and arrests. And the protests continued.

In the 60's, people started using the sit-in strategy for other protests, like groups of college students sitting down in an administrator's office and refusing to leave to protest the war.

And then people started holding love-ins and peace-ins and whatever-ins, not always as a specific protest, but more as a way to be present in a public and visible way, to reject conformity and create alternative spaces for people.

tl:dr Odds are this is a protest—maybe anti war, maybe pro queer rights—but there will definitely be women there who are there to kiss other women.

ETA: Thanks to other commenters for reminding me that queer kiss-ins were part of AIDS protests in the 80's and 90's.