i.varname1*varname3     creates dummies for categorical variable varname1 and continuous variable varname3: all interactions and main effects

        i.varname1|varname3     creates dummies for categorical variable varname1 and continuous variable varname3: all interactions and main effect of varname3, but no main effect of varname1

还有这个interactions 和main effect是什么意思啊，新手求助，谢谢