Yes, it's not very self-explanatory is it.¹

The red dot means that there is stuff to review.

More specifically, it means that some per-site threshold for stuff-to-review in a particular queue that you can review has been exceeded, and also you have not looked at the review queues for an hour.^2</sup

There is a Meta Stack Exchange post that mostly explains it. Most of the initial problems with the icon appearing too often or not going away soon enough have been ironed out. Basically, the red dot is trying to encourage you to click the review icon.³

¹ _{I'm going to take this opportunity to link to my favourite post about the redesigned top bar. I am in agreement with the last paragraph of this meta answer also.}
²_{On sites with very low activity, it really just means there is something to review}
3 _{Because that icon used to be a little button saying review and when they tested the new top bar without the text on Stack Overflow, everyone stopped reviewing.}

Yes, it's not very self-explanatory is it.¹

The red dot means that there is stuff to review.

More specifically, it means that some per-site threshold for stuff-to-review in a particular queue that you can review has been exceeded, and also you have not looked at the review queues for an hour.^2</sup

There is a Meta Stack Exchange post that mostly explains it. Most of the initial problems with the icon appearing too often or not going away soon enough have been ironed out. Basically, the red dot is trying to encourage you to click the review icon.³

¹ _{I'm going to take this opportunity to link to my favourite post about the redesigned top bar. I am in agreement with the last paragraph of this meta answer also.}
² _{On sites with very low activity, it really just means there is something to review}
3 _{Because that icon used to be a little button saying review and when they tested the new top bar without the text on Stack Overflow, everyone stopped reviewing.}

Yes, it's not very self-explanatory is it.¹

The red dot means that there is stuff to review.

More specifically, it means that some per-site threshold for stuff-to-review in a particular queue has been exceeded, and also you have not looked at the review queues for an hour.^2</sup

There is a Meta Stack Exchange post that mostly explains it. Most of the initial problems with the icon appearing too often or not going away soon enough have been ironed out. Basically, the red dot is trying to encourage you to click the review icon.³

¹ _{I'm going to take this opportunity to link to my favourite post about the redesigned top bar. I am in agreement with the last paragraph of this meta answer also.}
²_{On sites with very low activity, it really just means there is something to review}
3 _{Because that icon used to be a little button saying review and when they tested the new top bar without the text on Stack Overflow, everyone stopped reviewing.}

Yes, it's not very self-explanatory is it.¹

The red dot means that there is stuff to review.

More specifically, it means that some per-site threshold for stuff-to-review in a particular queue that you can review has been exceeded, and also you have not looked at the review queues for an hour.^2</sup

There is a Meta Stack Exchange post that mostly explains it. Most of the initial problems with the icon appearing too often or not going away soon enough have been ironed out. Basically, the red dot is trying to encourage you to click the review icon.³

¹ _{I'm going to take this opportunity to link to my favourite post about the redesigned top bar. I am in agreement with the last paragraph of this meta answer also.}
²_{On sites with very low activity, it really just means there is something to review}
3 _{Because that icon used to be a little button saying review and when they tested the new top bar without the text on Stack Overflow, everyone stopped reviewing.}