Q: Newbie Macbook Air

YessMP wrote:

 

, there is dialog box "Skype is an application downloaded from the internet. Are you sure want to open it?". What should I do?

 

That is normal.

Choose "Open".

For more info:

https://support.apple.com/en-us/HT201940

If you open System Preferences and select the Security and Privacy tab you can set the security level you prefer for downloaded software. (You may have to click on the General tab to see it. Unlock the dialog by clicking on the lock icon. See below:

To download the Skype application you must have switched your security to the same I have - to accept programs from registered Apple developers and the Apple store. Select anywhere at your own risk. The idea of the second selection is that Apple provides registered developers with a signed certificate that provides a level of security. Applications that don’t have that certificate embedded won’t run unless you have the third option selected. Presumably bad guys don’t want to register with Apple to get a certificate. The system isn’t perfect but does let third party developers a way to provide software outside the App store while providing us users with a measure of security.

Best advice is to keep this setting at one of the first 2 settings. Download software only from the developers own Web site to avoid malware and adware which is added by some software download sites. And download only from reputable developers (and this advice is tough - who is reputable is something you’ll have to decide on your own though people here will certainly warn you about developers we believe aren’t trustworthy.