Many of us use Gmail through a local email client like Outlook or Thunderbird. Google has good security but in order to use some clients, we have to set Gmail to allow ‘less secure app access’.
Google is turning that option off from the end of May. It claims that many people just sign into their accounts with their email and password, which are frequently stolen in data breaches. You can still use Gmail through a web browser, so not all is lost in the short term.
To get access back for your preferred email client, you may be able to ‘sign-in with Google’, which is what Google would like.
Other methods include using an app password. This is a long and unique code and makes things more secure. Login to your Google account, click your image and ‘manage your Google account’, then choose ‘security ‘on the menu on the left and in Signing in with Google’, switch on two-stage verification. Follow the instructions and then return to the security page, which should now have an option for ‘app passwords’. Choose ‘mail’ whatever email program you use. Now choose the device that you will access Gmail on, for example ‘Windows Computer’ and press ‘generate’ to get a 16 character password. Now open your email client, like Outlook, and put the 16 characters in as the password. You should now be able to work with Gmail. Remember you need a different code for each device, like your phone.
Here’s a short video: https://www.youtube.com/watch?v=cINrnBkm2eI