Its a Hindi linguistic placeholder for usually things, moments, instances, descriptions in general conversations.
Person B who, is obviously startled, can say " Yaar yeh kya thassa mara tu ne...dekh ke chala na??"
2) You are standing at night and you spot something weird in the sky and remark " Oye...yeh kya thassa hai?" or "Abe kya hai yeh?? Badaa ajeeb thassa hai yaar"
3) You try and appetizer which you find it to be extremely scrumptious, you can remark " Tagrha thassa hai yeh" or " bhai thassa achha hai, try maar"
4) You are introduced to someone new and you find that person to be weird. You can say to your friends " Guys, lets not hang out with this dude again...iska thassa theek nahin hai" or " Bada ajeeb thassa tha yeh...kyon bulaya ise??"