[simon]

What does "OSINT" mean and/or stand for?

As James said, open source intelligence. In a nutshell, using openly available information like images / video from social media, satellite imagery, etc to build a picture of what's going on.

For example, with what was going on in Russia in the last few days, OSINT played a major role in verifying what was going on. Images of the convoy could be geolocated based on roads/landscape features visible (compared against mapping, satellite imagery and Street View), and as it happened overnight approximate time could be ascertained by how far through sunrise it was, etc. Progress of the convoy through Russia as well as what equipment was in it could be verified.

It was possible to say with relative certainty that Wagner's claims were credible due to the volume of new, previously unseen imagery that all matched in terms of time of day, time of year, the same clearly identical vehicles between images.

There's a huge ongoing effort on social media to geolocate and verify images in conflict zones. For example:
twitter.com 

This is an example of the information that was gleaned during this weekend's events:
twitter.com