That's your left mouse button, primary mouse button 1. 2 would be right click, 3 would be middle click from the scroll-wheel - if you have one; you can also set both 1 and 2 buttons to act as a middle-click, usually for pasting clipboard text.
Can you right click the app in question on the task bar, and choose quit? If not, in System Monitor - highlight the app in question and right click, down at the very bottom - kill. That should end all processes that is currently open --
Be aware - like with my screenshot and Firefox; there are many sub-processes under Firefox. For this example: right click on Firefox (main process), select 'kill' at the bottom of the context menu, and that should stop all processes including sub-processes. If it doesn't - I would reboot at this point.
Also definitely run some updates through either Software Updater, or terminal with sudo apt update && sudo apt upgrade. Much of my fresh installs don't work - update the system, works fine. There might be some revisions that mitigate these issues you're seeing with an update.