今天早上，我的屏幕右上角出现了这个图标。点击或悬停都不起作用。我不知道它应该告诉我什么。有什么想法吗？
@pLumo：我知道这是一个愚蠢的问题，但是为什么图标本身不这么说呢？
更新：这是请求的输出：
$ sudo apt update
Hit:1 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:2 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:3 http://security.ubuntu.com/ubuntu bionic-security InRelease
Hit:4 http://ppa.launchpad.net/appgrid/stable/ubuntu bionic InRelease
Hit:5 https://download.sublimetext.com apt/stable/ InRelease
Hit:6 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:7 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:8 http://ppa.launchpad.net/jconti/recent-notifications/ubuntu bionic InRelease
Hit:9 http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu bionic InRelease
Hit:10 http://apt.postgresql.org/pub/repos/apt bionic-pgdg InRelease
Reading package lists... Done
Building dependency tree
Reading state information... Done
1 package can be upgraded. Run 'apt list --upgradable' to see it.
$ apt list --upgradeable
Listing... Done
snapd/bionic-updates 2.45.1+18.04 amd64 [upgradable from: 2.42.1+18.04]
N: There are 3 additional versions. Please use the '-a' switch to see them.
$ apt list --upgradeable -a
Listing... Done
snapd/bionic-updates 2.45.1+18.04 amd64 [upgradable from: 2.42.1+18.04]
snapd/now 2.42.1+18.04 amd64 [installed,upgradable to: 2.45.1+18.04]
snapd/bionic-security 2.37.4+18.04.1 amd64
snapd/bionic 2.32.5+18.04 amd64