What is the Left Mouse Button?
The left mouse button (abbreviated LMB) is the mouse button at the left side of the mouse, which would be under the index finger for a right-handed user. It's assumed to be the primary mouse button by default.
When someone says "click" or "drag and drop" they mean to do so by pressing the primary mouse button, which they assume to be the left mouse button. That's why we say "click" and "right click" normally instead of "left click" and "right click." Users who are left-handed can switch the primary and secondary mouse buttons in their operating system settings. In doing so, the left mouse button behaves as the right one and vice-versa, e.g. the left mouse button would open the context menus.