What does RHS/LHS mean?

According to this wiki page synfig.org/wiki/Convert#Add,

RHL = Right Hand Side ?
LHS = Left Hand Side ?

But I still understand what is the meaning of RHS or LHS, Could anyone help? Thanks.

Yes, Right hand side and left hand side.
I don’t know if it’s any way significant other than just a way to differentiate between the two parameters.

For the add valuenode, there’s not any difference between RHS and LHS. It’s used there for consistency with some other conversion types, such as subtract. Subtract is “LHS - RHS”, where LHS is on the left of the minus sign and RHS is on the right.