Once there are an variables that could effect the results of meta analysis as follow up periods( variations). How can interpreted results of meta regression or do moderator analysis to identify effect of this moderator on outcomes effect?
Basically what you are asking for is the interpretation of a moderation effect in meta-analysis?
The aim of moderator analysis (or meta-regression) is to explain dispersion of effect sizes, i.e., heterogeneity. So first make sure that there is heterogeneity to explain. If this heterogeneity is explained by one or more of your moderators, you can argue that the effect size (=dependent variable) varies with the level of the moderator (=independent). Example: if the effect size varies with the income level in a countries, you can show that with higher income, the effect of X on Y is also larger.
Note that you have not tested for these effects in experiments and that you cannot draw causal inferences on this. However, it is a very good reason to execute a future study that controls for these effects.
For more info, you can also check a text we wrote on 'how to interpret results from meta-analysis', including a section on Moderator Analysis.