As defined by Investopedia.com and acceleration clause is, "A contract provision that allows a lender to require a borrower to repay all or part of an outstanding loan if certain requirements are not met. An acceleration clause outlines the reasons that the lender can demand loan repayment."
But I think a more important question is why do lenders have such a provision. In a normal real estate transaction where the buyer is getting a loan to pay for their new house, the bank/lender has the buyer sign a promissory note. In return the buyer allows the bank/lender to take the house as collateral. This collateral is the mortgage. Now if a buyer for some reason wants to do a land contract or in some way transfer title to the home then the lender loses their collateral. Of course they don't want the lendee/home owner dong this so the have the acceleration clause to protect themselves in this event. If you own a home and are about to do something that may trigger the acceleration clause it would be best to consult with a real estate attorney. This is only one reason a lender/bank would exercise the acceleration clause.
All the best,
Jeffrey Paeltz, ABR SFR