A keystroke is typing one character on a keyboard (not stroking your keyboard like a cat). Every time you hit a key, you perform a keystroke. Therefore, 5400 keystrokes in one hour means hitting 5400 keys in one hour, or 90 keys a minute (5400 ÷ 60 minutes).
Sometimes keystrokes per hour (KSPH) or keystrokes per minute (KSPM) are used to measure typing speed instead of words per minute (WPM). After all, typing the word "hi" 50 times doesn't take quite as long as typing "Nebuchadnezzar" 50 times.