When I click on my terminal, strange characters appear

You have somehow activated the terminal's mouse tracking mode. This is usually used by terminal applications which accept mouse input (like vim), which can interpret these character sequences as mouse events. The shell does not support mouse input, so it displays these sequences directly.

You can turn off mouse tracking mode by running the reset command in the terminal.

I just had this issue and going to view > Allow (or disallow) mouse reporting will solve it.

When the Terminal (well, one of its ttys, anyway) is not accepting keyboard input in a reliable manner and needs a hard reset, either:

A.) hold down the ⌥ and ⌘ keys, and tap the R key [option-command-r]; or:
B.) Invoke the same action under the "Shell" dropdown menu of Terminal.app: