It means the website is being delivered over a secure connection – which is good. But… beware of complacency.
All the green padlock means is that the connection is secure – it doesn’t give any legitimacy to the website. If, for example, you get an email asking you to reset your HMRC password and the link (which you shouldn’t click on anyway) takes you to a website with a green padlock, don’t assume that it’s safe.
That little padlock is an indicator that you can safely submit your personal / financial details to a website – but ONLY if you trust and know where it is going.