The intercept is the prediced value of the response when all predictors are 0. SEM usually stands for "standard error of the mean", which is a measure of how sensitively the likelihood of the data drops around its maximum.
If you mean SEM = structural equation modelling, then the answer is here: https://stats.stackexchange.com/questions/205752/what-is-an-intuitive-definition-explanation-of-an-intercept-in-sem