打卡：post on the internet about finishing a routine, such as work out, study, or any habits that take effort to keep.
打卡 originated from work locations that have card machines to record time you get to work and leave. So 打卡 originally means finished a day's work. Your boss will supervise you, to check if you 打了卡 or not. If not, you'll get some punishment.
About 打卡 in web posts. As a habit that takes some effort to build and keep, one feels like that himself alone is not sufficient: he wants to be supervised and urged by others. So people with the same need grouped together and make an online forum. There they post their progress every day. Whoever fails to report his progress will feel shame. In this way, they supervise each other if they finished their daily work out or not. And the routine of post progress each day is very similar to insert the card into the machine at the workspace. So people call this action 打卡：
Did you exercise today? Come and post your achievement!
(On a web post) I come to check in: I completed this set of workout, today. You guys carry on.
I forget to check-in yesterday, that I did a set of leg work out yesterday. Here I make this post to make up.
(Can also be understood as) I forget to do the leg work out yesterday. I did extra leg exercises to make it up.
The action of 打卡 is the action of posting on the internet. And the post has to be about finishing something to make it a real 打卡.
Another meaning of 打卡 is to check in to a place, usually a restaurant, a store, or any place of interest.
Check in to a famous bobble tea store.
打卡 Pikes Peak.
Pikes Peak, visited.
I am not sure how it originated. It might be from the register machine of hotels, I guess.