Investopedia said that it is the action of selling stock to cash in on a sharp rise. This action pushes prices down temporarily. When traders are profit taking, the implication is that there is an upward trend in the security.
But why Friday? At least in the Philippines, traders seems to be going for a cash on this day because during the weekend we do not know if there will be a big bad news that will pop up and your cash is tied to a certain stock it might be hard for you to sell your shares on Monday. Also, the US Market is one day behind than us. When we end our market on Friday, there is still 1 more trading day for the US and as we all know greater chances that our market will follow the sentiment of the US market so if on the last trading day of US there will be a big dip, your shares on PSE might be affected. Some traders will be contented to cash-in their position and just get in the next week rather than placing your money for the weekend with uncertainty.