To flag something up is to emphasise its importance, or to mention it so that others know about it.
At the meeting Martin flagged up the issue of late payments. He said that we hadn't paid our suppliers for three months.
There are a couple of points that I'd like to flag up today: one is the increasing number of customer complaints and the other is the problem with our computer system.
If you are flagging, it means you are becoming weaker and running out of energy.
I worked really hard from 7am until lunch, but after lunch I was really flagging!
The colourful triangular flags in the picture are called 'bunting', and are often seen at street parties and weddings in the UK. The flags have a long history in the UK, though they have seen a particular return to popularity after the Royal Wedding of Prince William and Catherine Middleton in 2011.