Tosca will calculate and report an RMS error for your control points. You can decide if the error is acceptable or if you want to try again. Set the autosnap. Autosnap will cause a newly digitized node to snap to the position of an existing node if it is within a specified distance. Choose 'Set autosnap' from the menu and set it to the value of the actual or acceptable RMS error.
Nodes are the end points of any kind of line or line-like feature.
Set the tolerance. Tolerance defines the distance from an existing point in which additional points cannot be digitized. For example, if the tolerance is 10 you will have to be at least 10 distance units away from the previous point before you can add another point. Choose 'Toler' from the left middle menu by typing 'T' and following the instructions. Choose a tolerance about twice the autosnap distance.