Here is my explanation.
Resonance is the ability of a molecule (or ion or free radical) to be represented by more than one reasonable Lewis structure that differ only in the location of the π electrons
An example is the acetate ion, which has two equivalent resonance structures.
In the English language, mesomerism is an old term for resonance.
The mesomeric effect is the electron-donating or electron-withdrawing effect of a group because of resonance.
For example, carbon 3 in propenal has a slight positive charge because the aldehyde group withdraws electrons through resonance.
The aldehyde group has a –M effect.
(From Difference Between)
Similarly, carbon 4 in 1 methoxy-1-3 butadiene has a partial negative charge because the methoxy group donates electrons through resonance.
The methoxy group has a +M effect.