Specify on command line
If you are correctly watching the command line - a common basic mistake if you aren't - you will often see the word specify in the instructions.
Typically it means a left mouse click. To start a line for example from "Specify first or next point" is a left click.
Sometimes in the circle command for example you are asked to specify radius which is asking for a value.
However you can provide 2 left clicks and CAD will take the distance between those clicks as the radius required.
Sometimes you will see specify distance, for example in the offset command. Again a value is required but because of the specify rule you can provide 2 left mouse clicks and offset will read the distance between them. Use this technique to copy an existing offset distance.