to show your ticket at an airport so that you can be told where you will be sitting and so that your bags can be put on the aircraft

Passengers are requested to check in two hours before the flight. 乘客应在飞机起飞前两小时办理登机手续。

You can save time by checking in online. 你可以用网上签到以节省时间。

to contact someone by making a phone call, short visit, etc., usually in order to make sure there are no problems or to tell them that there are no problems

My son checks in regularly with me when he's travelling. 我儿子在外旅游的时候会经常打电话给我报平安。

to give your bags to airport employees to be put on a plane for you, rather than carrying them on and off the plane yourself

Will you be checking in any luggage, madam? 女士，您有行李要托运吗？

Those who need to check in bags must be patient and prepared for security searches. 需要托运行李的人一定要有耐心，为安全检查做好准备。